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»	New	Full range Melquiades implementation..	Solved...	Melquiades Amplifier Forum	81	569580	02-23-2011
»	New	Melquiades For Dummies™ - step by step...	Amp still open...	Melquiades Amplifier Forum	28	214687	08-29-2007
»	New	Building Melquiades: Chronicle of full-range..	VR2 issue leading to jump in current on 6C33 tube...	Melquiades Amplifier Forum	44	435536	06-09-2006

02-04-2014

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decoud
United Kingdom
Posts 247
Joined on 03-01-2008

Post #:	81
Post ID:	20523
Reply to:	20522

Milq high pass

The coupling capacitor is a first order high pass filter. I have .68 in there for there for this purpose. Obviously the slope will be shallow, so precision in the value will not be critical.

02-04-2014

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	82
Post ID:	20526
Reply to:	20523

Coupling capacitor

Daniel, if you have start, the igniting voltage, on the gas tubes 197V then it is perfect. If the tubes do maintain stable 150C then leave it as it. Each of the gas tribes has own stabilization voltage. If one of them has let say 148V and another 150.3V then you might have not enough turns in your adjustable resistor to set the input voltage to zero, So, juts change one of the gas tubes or switch them between each other. As you will set the gas tube and lock the operational parameter it will be good for many years.

The high pass filter for Milq is very simple. As “decoud” said the coupling capacitor is a first order high pass filter. Set it for whatever frequency you want. If you need sharper filter, let say second order then you can drop a coil from grid to ground, introducing another first order via RL filter. I would not do it and use just filter in coupling capacitor and if you need more than I would do it at speaker level, you do not run DSET and all your filtration at speaker level. The coupling capacitor filter will be nice to protect the OPT from any LF saturations.

Rgs, Romy

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

04-03-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	83
Post ID:	24790
Reply to:	20526

Bias

Hi Romy

I have built a Melq prototype and need some help with the bias.

This is my first tube amp build so my understanding is not great.

All parts values were used as per the 4.4 schematics. Rectifier bridges were built with Infineon SDT12S60 diodes. I was getting slightly less voltage on DC side however I’ve ordered my power transformers with multiple primary taps so I was able to adjust it.

My gas valves stabilise at 146.1v and 145.6v.

When I set 0v dc at the input via VR1 I get - 4.3v on the driver grid. The B+ on the first tube is 189v.

Should the grid bias voltage change when I adjust VR1? In my case it does – if I keep increasing dc on inputs I can get to -1.5v on the grid and around 60v on B+.

Everything seems ok but when I check the V1 voltage at B (146.1v) on the positive bias line the dc on the input suddenly jumps to ~0.47v, the grid voltage drops to -3.5v and B+ goes down to 160v. Now if I test the negative bias voltage at C everything goes back to how it was before.

The other thing is that when I switch the bias power off the dc on the inputs goes up to +20v.

The DC on the inputs also drifts ~+-50mV.

Is there any difference which side legs you connect the VR1? I connected mine so I get 0 to 20k. If I was to connect it the other way round then I will get 0-5k-0.

Where should I take the measurement for the V1 current draw? Do I connect my multimeter in series between R11 leg and tube anode?

The amp is very quiet when I test it with my old ipod however if I connect it to my preamp or dac it’s noisy so I need to double check my grounding probably.

The output stage seems fine. I get ~202v/ ~195ma. ~-78v on bias. I use Lundahl ll1627 amorphous transformers wired in D. My fane 8m is 8r and s2 16r.

I tested the amp with midrange and upper bass and it sounded very promising. It’s very fast and harmonically rich.

In your correction thread you recommend: 12) Put two 10R, 5W resistors (in each leg) between the rectification bridges and secondary of the power transformers. Is this necessary with my bridges ? I haven't added them yet.

Thanks!

04-03-2018

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	84
Post ID:	24791
Reply to:	24790

God, it was so long time ago!!!!

G wrote:
Should the grid bias voltage change when I adjust VR1? In my case it does – if I keep increasing dc on inputs I can get to -1.5v on the grid and around 60v on B+.

Nope, it should not, well it is and it is not. Are you sure that you put the VR1 resistor in positive side of bias? Let me to explain. What I typically do to experiment is shorting the amp input and disconnect positive bias. The resistor R5 in this case acts as a bias resistor separating voltage between negative supply that goes the tube grid and ground. Let pretend that you have 4V on the driver grid while input is shorted. Now the certainly is balanced perfectly. If you un-short the amp input then bias will move but adding the positive supply on the left side of R5 resistor and setting 0V in there you introduce a virtual ground for the left side R5 and the bias in case you have 0V at input should be identical to the bias when the inputs are shorted. Under normal circumstances you would never adjust 0V at input, I did it one and over year is stays 0V. Even change of the driver tubes do not effect it. The only time you would need to do some adjustment would be what you change gas tubes as all of them stabilize slightly different voltage. Generally this topology might be recognized as “virtual battery”, where R5 resistors acts as voltage emitter because it has 0V on left and minus 4V on right. Anyhow, if your bias voltage move too MUCH when you adjust VR1then there are two possibility:

1)    You put the VR1 accidently on negative side, between R10 and C5, or somewhere in that negative supply.
2)    Your pream has very high output impedance.

Bothe on the case above are not good but they also are not bad. If you adjust 0V at input then you do not care about anything and you filter out all external factors.

G wrote:
The DC on the inputs also drifts ~+-50mV.

It should not be this way. If you say that it drifts ~+-5mV I would say that it is too high but let it to drift. The 50mV is too much. Switch the gas tubes, I think one of them are faulty. In case you use a direct coupled preamp make sure that it does not send any DC drift to its output.

G wrote:
Is there any difference which side legs you connect the VR1? I connected mine so I get 0 to 20k. If I was to connect it the other way round then I will get 0-5k-0.

Sorry, I do not know what VR1 you use and I do not know the specific of it’s connection. You can use in there any good multi-turn in order to set 0V at input. If you have not enough turn to do so then slightly adjust R9 or R10. The better would be is to change the gas tubes, put positive to negative, or to get a different set all together. They stabilize anywhere from 144V to 156V and a half volts of stabilization differences would reflect good 10K in VR1 resistor. Do not hold me responsible for exact numbers I juts pull it out of my ass now. Anyhow, flipping the tubes between the positive and negative bias supple always works for me.

G wrote:
Where should I take the measurement for the V1 current draw? Do I connect my multimeter in series between R11 leg and tube anode?

You did not mean R11, it is juts next to Point “B”, you rather meant R2, which is point “M”. Yes, you can put a multimeter before R2 and to measure current. It should be around 25-35mA, depending of the tube. A better solution in my view is to put anywhere in series with R2 a good quality 1R resistor and then to measure drop of voltage before and after resistor. You will be using multi-voltmeter in this case, you can add and remove it in real time, or live it permanently if you want and the voltage you will be getting will be your current in mA. You can even measure the voltage drop over R2 resistor but then you need to calculate the current by Ohm’s law.

G wrote:
The amp is very quiet when I test it with my old ipod however if I connect it to my preamp or dac it’s noisy so I need to double check my grounding probably.

Here is where you need to figure it out yourself. I do not think that anybody would help you debug grounding loops remotely.

G wrote:
I tested the amp with midrange and upper bass and it sounded very promising. It’s very fast and harmonically rich.

I know. Please contact my wife and suggest her that I am not an idiot. :-)

G wrote:
In your correction thread you recommend: 12) Put two 10R, 5W resistors (in each leg) between the rectification bridges and secondary of the power transformers. Is this necessary with my bridges ? I haven't added them yet.

Nope it is not necessary. I do remember that I did those experiments but I do not remember details or results now.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

04-05-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	85
Post ID:	24793
Reply to:	24791

Further troubleshooting

Thanks for your reply.

I have double checked everything and my VR1 is connected on the positive bias side. With the input shorted I get -4.2V on the grid and it stays the same after I unshort it and adjust VR1 to 0v. Yesterday voltage was drifting +-10mV with nothing connected at the input but as soon as I connect my preamp it drops to 0V and stays there. So should I be measuring DC with the preamp connected and just ignore the voltage drift when nothing is connected?

The big problem that I have is when I switch the amp off the voltage on the input creeps up to around 55v over 1 minute and then slowly starts dropping down as the last positive power supply cap - C8 starts to discharge. The last negative bias power supply cap C9 discharges quite quickly into R17 but C8 stays charged up. Something is not right.

What are the R13 & R14 – these should not be connected to ground?

V1 on the schematics is the positive supply gas tube. I guess that the process for measuring the gas tube current draw will be the same. Please let me know if below is correct.

Thanks

Gintaras

04-05-2018

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	86
Post ID:	24794
Reply to:	24793

Until next week....

Gintaras, we are in Mexico for the next two weeks. As we return I will revise it and reply to you, sorry I'm not ignoring you, I'm just I'm trying to be unplugged.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

04-12-2018

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	87
Post ID:	24804
Reply to:	24793

You are OK.

Gintaras, basically it is not bias voltage problem but voltage drifting +-10mV with preamp disconnected. The 10mV is absolutely irrelevant voltage. You see, the gas tubes are voltage stabilizers but they are not as precise as the SS stabilizers. They stabilization is greatly contingent of the current you drive through them and even at optimum current of I think 15-25mA it is still not perfect. Also, it would be different if the gas tube not broken in as well as wary from tube to tube. If you have stable voltage when the preamp is connected then I do not think you need to worry about it. I do not know what preamp you use. Some DC preamps has DC offset secretly at outputs that might kill your drifting. What you need to understand is that when the preamp is connected and the left side of the grid bias resistor is shorted to ground by preamp’s output impedance then the positive bias become irrelevant. In fact you might even pull the positive gas tube out and shot down the positive supply. Do not disconnect the preamp wires while you are doing this as it will send a huge pile to you al and will blow your speakers. So, the positive supple is what you need only for amp to work stable in disconnect more. You can even not to measure the voltage at the amp’s input. Make it somewhere near 0 and then plug in and out the cable from your preamp. You will hear a characteristic clicking from speakers. Then adjust the VR1 to the point when connecting and disconnecting preamp will produce no clicks.

The voltage on the input creeps because your positive and negative supplies are discharging with different rate. Hypothetically, you can adjust it by bleeders R13 and R14 (be careful as they set the minimal current over the input chokes) but I find it absolutely no necessary. The voltage at input creeps only with disconnected preamp and it is absolutely irrelevant what voltage is there if the circuitry is not complete.

The measuring the gas tube current draw you depicted correct, the current is set by VR and R9 resistors. I do not think you should worry about it. If I am not mistaken it should be 15-25mA but I know that the schematic shall be well calculated. If you would like to be very anal about it then you can fix for your type of gas tubes the best current, when it produce the best regulation, but I would not do it for positive tube as the positive supply is less relevant. You need a gas tube there to have “soft buffer” that is about it.
I think you are in good shape and if I were you I would not worry about anything.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

04-13-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	88
Post ID:	24806
Reply to:	24804

Great!

Thanks Romy.

Ok. I have installed a 30k resistor on the input and it reduced the drifting when the preamp is not connected. If my preamp is connected then I do not get any drifting at all and input voltage stays at 0v when I switch the amp off. No ticking when I plug/unplug the preamp. My gas tubes were drawing ~10.5mA so I reduced the R11 and R12 to 1.8R and now they draw 21mA. The gas tubes now regulate much better the voltage on both tubes got very close– 153V on positive and 152.8V on negative. Before the difference was around 1V.

Should R13 & R14 be connected to ground?

I also checked the current draw for the first stage and I am getting around 13-14mA which seems a bit low if I need 25-35mA. Is this enough? My voltage on M is exactly 400v. Voltage on A is 193V. Bias -4.4v.

I can probably start putting everything into a case.

Regards

Gintaras

04-13-2018

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	89
Post ID:	24807
Reply to:	24806

The R5 is the key.

Gintaras,

yes, the 10.5mA was a bit too little for gas tube, 21mA. If you look at the stability of voltage regulation graph for these tubes then you will see an ark with 15-20ma being atop.

I do not understand why you keep asking me R13 & R14 connected to ground? Of cause they must be connected. If they not then the whole bias supple will not work properly. They are loading resistors and bias supple filtration work loaded to them. If they are not connected to ground then your input choke filters are loaded only to the gas tubes which draw too little current. As you see the same is done in case of R23. The reason for it that the inductance of input choke that I use must be tuned to the passing current, it call minimum inductance. If the current and instance do not tune then choke will not go provide voltage filtration and will not provide a proper current regulation. Google about “input choke minimal inductance”.

The current draw at the first stage is not good. If you have ~ 200V at plate, the 193V is fine, then you are fine. You want the tube pass ~ 30mA. If you have less then very slightly change you R5 resistor. Your -4.4v at bias is a bit too low, the schematic shows 3.4V and anything up to 4V is fine. Try to change the tube itself, might be you get a faulty one that does not develop current.

It is little stage and the voltages and numbers in the schematic are very accurate. Dafter six month of experimenting with it 20 hours a day I built 2 pairs of full range Milqs and a pair of 6 channels Milqs, I always hit the targeted voltage with no adjustments. Well, of course if I did everything right, which was not always the case. Anyhow, ground your R13 & R14, check everything, change the driving tube and if you still have no 25-30mA of current then change R5

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

04-13-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	90
Post ID:	24808
Reply to:	24807

25-30mA or 15-17mA

I double checked and everything seems connected ok. all parts values are correct. I have measured the voltage across R2 and it was 208V so 208v / 15k = 13.86mA. I connected 1r resistor between M and R2 and it was 14mA. also connected 1r resistor between cathode and ground and got the same 14mA.

next I changed the R5 with 20k trimmer. shorted input adjusted trimmer to get -3.5v and the current increased to 16-17mA but the plate voltage dropped to 160v. if I increase the bias to -4.4, plate goes back up to ~195v current drops to 14mA.

Also tried swapping 5 tubes but the difference was +-1mA

25-30mA seems like imposible. something is still not correct.

In some of your old posts you said that the current should be 15-17mA so I am a bit confused now as to which one is correct:-

“10) Now you need to adjust the amount if bias on the tube in order to have 200V on plate. You understand that amount of bios is irrelevant and it would depend from the given tube of yours. Averagely the minus 3.4V-3.5V will give you 200V and 15-17mA of plate current. Do not be too finicky about it – the current in the driver tube in THIS application is not really critical. Set plate voltage between 170V and 200V with whatever bias voltage it will be (let presume -3.4V+/.5V)”

“I usually get around a year out of 5E6P driving it at 17mA and 200V. I do not know how much I will get out of 5E6P at 35mA as I use them in the single-stage amps.“

G

04-14-2018

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	91
Post ID:	24809
Reply to:	24808

Ok, you got me here.

Gintaras, this is funny as in my last post I meant to write that I might not remember correctly and you need to look in my old posts. I did not write it for whatever reasons but what you did was right. Whatever I wrote in my old post was correct. My apologies that I mislead you now. Indeed, the current means not a lot in your configuration, when 5E6P drives 6C33C. The 6C33C is indirect heated tube and can’t work with plate current. So, you do not need current from 5E6P to push 6C33C into class A2. The critical current from 5E6P 35-40mA and anything under is fine. The 15-17mA would be a good zone of comfort.

If you do want go with higher current then you drop a little bit a plate resistor R2. I do not know if you need to. The change of plate resistor does impact sound but not because of current but because of other things. At the time I was making the Milq we had a lot of debate what plate resistor should be. Some people I know use very low value, 5K with lower voltage PS. They had the same current on the tube and slightly different current. I feel that high plate resonance is good for this tube. Again, I can’t recollect now any specific sonic changes I remember with using high plate resistor vs low plate resistor. I assure you that using a resistor for 1-2K lower you will not hear any sonic difference.
Another, purer, way to get more plate current is supply different negative voltage to the 5E6P grid. You can do it by reducing R6 resistor. This will not impact the 5E6P operation point.

Now about the 5E6P current and my 25-30mA misleading. I did not look at my old posts and was referenced to what I was using in my current multi-channel version of Milq.

http://www.goodsoundclub.com/PDF/6-Chennal_Melquiades_DSET_Amplifier.pdf

If you look at the schematic than you can see that channels E, F and D have in the negative (green) power supplies 100K trimers. I use the trimers to change plate currents and I usually stay with 25-30mA. The channels E, F and D are not the same as what you have as they are single channel where 5E6P drive speakers directly. You need current there but not so much in the channels where 5E6P drives 6C33C.
If you are uncomfortable with your plate current at 5E6P then you might do the same and put a trimmer after R10. After you find the best for sound current (do not go over 30mA as you might burn the tube) then measure the final resistance and put in there a permanent resistor. This supply to directly to the tube grid and the less cap you have in there the best it will be.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

04-16-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	92
Post ID:	24815
Reply to:	24809

Burnt fingers

Thanks Romy.

14mA I am getting vs 15/16mA is not a large difference and the amp sounds really good as it is so I will have a break from soldering as my fingers are all burnt and listen to the amp as it is for the moment.

Gintaras

04-25-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	93
Post ID:	24822
Reply to:	24815

Grounding

I am trying to come up with a grounding scheme before I start putting everything into the case.

Please have a look at the below drawing. I have tried to replicate what you did with your fullrange Melq however I am not too sure about the ground connections in yellow. Any advice on on the best way to connect them?

Thanks!
Melq grounding.png

04-25-2018

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	94
Post ID:	24823
Reply to:	24822

Yes, something like this.

Yes, something like this. This all common sense. Make the main grounding bus with some kind of ugly, very low Ga (1-6) wire with no isolation. Make sure it is copper, so main grounding bus should be a LOT of copper. It should be connected to amp’s chassis in one location, the RCA for the amp input would be the best. If you have think enough main grounding bus then the locations where you connect to it different devises will be less relevant. Ideally you would like to connect all devises to the input ground, in a single point. It is not practical however. A good thick main bus terminal will make it more convenient. I am not a reference in circuits assembling but using the techniques above I never had any ground loop problems in the circuit

The connection to the main’s ground is a tricky question. It should be connected of cause, not the last reason is the electrical code demands and the considerations of safety. The really is very different and would be contingent upon many factors: quality of your main grounding, the type and the grounding topology of electrical proxy devises you use, specifics of your particular intentions and many others. So, wire the main’s ground to the amps and it is highly like that you will eventify lift it by using a cheating plug. I did it with each single playback system I had.

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

05-31-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	95
Post ID:	24904
Reply to:	24823

Buzz off

The amps are finished. I had some minor noise and the cheater plugs did it they are completely noise free now which is very nice. The meters are a bit ugly so decided not to install them inside the case just added on top for now. Will remove them and add multimeter test points.

05-31-2018

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Romy the Cat

Boston, MA
Posts 10,184
Joined on 05-28-2004

Post #:	96
Post ID:	24905
Reply to:	24904

Thanks.

It is very nice and indeed and under normal circumstance you will not need any permanent multimeter. It is for sure nice to have it when you put a new 6C33C in use as plate current run wildly while the new tube is burning in. It is still OK to do it with no multimeter. What I do is what a new 6C33C is in I set the plate current for 100mA and let it to burn for a half day. Then the run out of the current become less and less and I set it to ~150-170mA for a fully heat up tube and run it is it for another few days of listening. After 20-50 hours on the tube I set the full power on it.

I do feel however that multimeter is very useful what you have multiple channels and small kids in house. They tend to turn anything turntable and if I arrange the controls on Milq after I become a dad then I would put controls at very different locations.

Anyhow, it is kind of special pleasure to see the Melquiades built out there. It is not the ego things, just a pleasure. It is 6AM in Boston, the Sexy Amy is sitting in our library, playing with her computer and drinks her morning coffee. I called her and showed to her the picture of your Milq, with Bruckner records on backdrop and my schematics. She was very distinctly purring….

PS: a question. I see that the internal wall of the amp covered with some kind of foil. Was it intestinal? Is it a regular Aluminium foil? Are you planning to do extras shielding by it?

"I wish I could score everything for horns." - Richard Wagner. "Our writing equipment takes part in the forming of our thoughts." - Friedrich Nietzsche

06-01-2018

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G
Posts 8
Joined on 03-31-2018

Post #:	97
Post ID:	24906
Reply to:	24905

PeaceMAT XS™

Thanks for sharing the schematics and everything else on this site.

The aluminium foil shielding is accidental. It's 2.5mm sticky elastic butyl pads with aluminium backing used for car damping. I used it to damp the case and they were very effective.

https://www.ebay.co.uk/itm/Silent-Pads-2-5mm-10-LG-Sheets-Pack-Car-Van-Deadening-Sound-Proofing-Damping-Mat/262418079335?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649

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	Target	Threads for related reading	Most recent post in related threads	Forum	Replies	Views	Started
»	New	Full range Melquiades implementation..	Solved...	Melquiades Amplifier Forum	81	569580	02-23-2011
»	New	Melquiades For Dummies™ - step by step...	Amp still open...	Melquiades Amplifier Forum	28	214687	08-29-2007
»	New	Building Melquiades: Chronicle of full-range..	VR2 issue leading to jump in current on 6C33 tube...	Melquiades Amplifier Forum	44	435536	06-09-2006