My JCM800 6V6 build ...a few issues

[Shack]

So I built Rob Robinettes jcm800 in my Princeton reverb cab. The amp works , kinda...I have high frequency oscillation when playing through it, but at the moment , I want to make sure the tubes bias as they should. I know that the layout is already tested and proven, but I ran into a few problems.

I cant measure plate dissipation because I cant get a good voltage drop reading and the transformer shunt method isnt helping, so I put a 1 ohm resistor from cathodes to ground and measured the negative voltage on the grids. I have 195 ohms from plate to center tap on each tube.

I can only get like 40mv(ma)for both tubes across that resistor, and -45 volts on grids with bias pot maxxed out, and there is alot of noise when I max the trimpot and master volume ( its a trainwreck type 3)

[shooter]

I can only get like 40mv(ma)for both tubes across that resistor, and -45 volts on grids with bias pot maxxed out,

you gave EACH tube a 1 ohm?  (cathode to ground)

what's your plate VDC, -45 seems REAL cold for 6V6

[sluckey]

I have an AB763 6V6 amp and a Plexi 6V6 amp. Both have -36V on the grids, about 400 on the plates and 25mV on each cathode. I have a separate 1Ω resistor for each 6V6.

You provided a grab bag full on numbers, but you can't put any of them together to say how your amp is biased. I will say, -45v on the grids is too much for 6V6s. That will likely be around -35v when you get this sorted.

[Shack]

I will put a second 1 ohm resistor tomorrow, but that is where im at.....and dont know why

[Pietro]

if the amp is oscillating severe, than there is no reasons to try and set the bias.

oscillation wil prevent you from doing just that. So that has to be adressed first.

See if you have any input signal wires
that are routed wrongly. Are all signal wires shielded?

[Shack]

I can work out pre amp issues later, but the power and bias sections need to be worked out first, in my opinion

[Shack]

Sluckey, I cant get my negative voltage down to -35 atm.....the bias circuit isnt allowing me to , without making the balance circuit dangerous to one of the tubes, I think

[sluckey]

Sluckey, I cant get my negative voltage down to -35 atm.....the bias circuit isnt allowing me to , without making the balance circuit dangerous to one of the tubes, I think

Change that 470Ω 2W to a 1K 2W. Any better?

[Shack]

So with the 1k 2w , I have it down to -38.8, Plate voltage is 401 and 400....and I havent ran the 2nd one ohm resistor, but the 2 cathodes combined read like 52 ma

I am going to retrace this again and see if anything shows itself

[sluckey]

Plate voltage is 401 and 400....and the 2 cathodes combined read like 52 ma

Assuming the two tubes split equally at 26mA each, that puts dissipation at 400V X .026A = 10.4 watts. I call that good.

However, if the bias pot is turned all the way to one end to get that -38.8V, I would increase the 1K to 1.2K or even 1.5K.

BTW, you cannot set the bias balance pot accurately until you put one 1Ω resistor on each cathode. Only then can you measure the voltage on each cathode separately and set them to be the same. My preferred method to set the balance would be put one meter probe on pin 8 of one tube and put the other probe on pin 8 of the other tube. Adjust the balance for zero millivolts reading on the meter.

[Shack]

Well, hooking up other 1ohm resistor, and I plugged in a solid state rectifier ( to make everything more like the layout) I am able to see that there is something going on between the balance pot and the master volume....The mv in both tubes seemed to go in the same direction when I turned the balance pot.....then I realized that the master was turned down, and when I turned it up I had 30-40 ma and I now balanced them to 30 each.....

Also the high freq oscillation comes on by turning up the master volume and/or the bias pot

Everything does seem to be hooked up right...i would have to figure out how to get a picture up.

[sluckey]

Of course that crossline MV is gonna mess up the bias balance. It shorts the two grids together! This is a really poor design. Looks like someone tried to put all the bell and whistles into this schematic and never tested it. I suggest you remove that MV pot.

The hi freq oscillation may be due to incorrect NFB phase. Swap the OT plate leads to see if this cures the oscillation.

[shooter]

This is a really poor design

+1
tried it twice, removed it twice 

[Shack]

So an update.....I switched the OY wires around and that was the oscillation , usually it was louder so I didnt consider it.

I have been playing it, works good, but the bias is still a little cool....havent adjusted resistor yet.

left tube- 426pv       .023ma      201ohms       4.73 vd ......10.0W pd

right tube 426pv      .024ma      207ohms       5.04vd.........10.3W pd


the bias pot is maxed tho....the cathode res read 25.7 ma on left and 26ma on right tube

Thank you Sluckey 

[Shack]

Oh and also I wired in a normal MV like im used too

[DummyLoad]

if you must have it, cross-line MV needs to be uncoupled from DC part of power tube grids with another pair of caps. cross-line MV suck.
remove cross line MV and put put MV back where it belongs: hanging off the treble wiper.
bias balance is joke for a guitar amp and this plan has no protection if wiper of 25KL pot fails - yes, pots DO fail.
with 320V B+ to cathodyne, you really don't need heater elevation, but you've already wasted the ~$1.50 there.   


--pete


EDIT: well, i was about 30 secs  late to the party...

[Shack]

Yeah, I thought that about heater elevation, as ive never had an issue with heater hum .

I contemplated taking the balance part out because it seemed more complicated than it was worth, and I still dont have a good enough bias sweep, especially because this amp is wired for EL34 as well as 6V6/6L6.......

But it does work good, I should redo measurements with the GZ34 back in instead of the SS plug in, although I dont think the sound will change much.

[sluckey]

and I still dont have a good enough bias sweep

However, if the bias pot is turned all the way to one end to get that -38.8V, I would increase the 1K to 1.2K or even 1.5K.

[Shack]

LOL.....I know, im going to change it now, as I do have some oscillation , pretty loud in the high gain jack, I didnt hear it before over the roar of the guitar

[Shack]

Crap.....thats my pedalboard making that sound

[shooter]

Crap.....thats my pedalboard making that sound

 
use a signal generator, then you don't even have to move from bench, or add gremlins 

[Shack]

No Shooter...there isnt a reason for a signal generator .....its done, for all intents and purposes, but my pedal board and the high gain input isnt playing nice...I wonder if shielding all inputs would help that, because its my favorite sounding input, altho im paying through the other right now and its not bad....my OCD pedal makes noise through my other amp too.

[Pietro]

i remember somebody asking you if your inputwiring was shielded.

Always use shielded wire. Not doing so is a mistake people usually only make once.

[Shack]

I have had good luck not shielding wires, but ive never built an amp with higher gain, so I will probably shield them.... this amp is incredible, but there is noise between the high gain input and my board

[Shack]

and the volume and noise between inputs is too significant, what wold Dave Friedman do

[Tone Junkie]

 Go with the shielded wire It just makes life easier on the input. I never used the stuff until I started building high gain amps. I tell you these guys have brought me thru some real interesting builds.
and put your grid stopper resister right at the tube not on your board. Low gain circuits resistor on the  board no problem . High gain shielded wire and at the tube. just my two cents. about all its worth.
Thanks Bill