Ampeg V4 questions on cap value

[sgelectric]

If I'm not mistaken:


V4 V4b and VT22 amps all use 4 7027A power tubes for about 100W
V2 and VT40 amps use 2 7027A power tubes for about 50W


Regarding vintage production 7027A and 7027 tube types:
Do not confuse 7027A tubes with 7027 tubes. The plate voltage and dissipation ratings are significantly different.

I have used 6L6GC tubes in these amps without trouble and w/o circuit mods.

SG

[g1]

Do not confuse 7027A tubes with 7027 tubes. The plate voltage and dissipation ratings are significantly different.

Just to reiterate: there are no modern production 7027A tubes that meet the original spec. They meet 6L6GC specs and are actually re-pinned modern 6L6. If you don't need the 7027A extra pins, modern 7027A's are a waste of your money, use 6L6.

[DrGonz78]

So I got the JJ-6L6GC tubes, replaced tube socket, replaced C303/C305, resoldered the boards, and replaced the electrolytic caps on the bias supply. The resistors for the bias supply are well in tolerance and look visibly in good shape too. I am about ready to power up the beast without tubes and will use variac to reduce plate voltage, as G-One stated. All looking good but then I did some last minute checks. Found that D206(by the standby swtich & pilot light) was shorted. So I am replacing that with a 1n4007 diode as parts list states 1A 1000v for all those diodes D201-208 on the power board. This thing is missing the reverb tank and reverb tank wires are just flying around a bit willy nilly. The green wired one for the input side definitely was involved with what ever melted down in this amp, as it is all blackened in parts. I have to ask myself if somehow the reverb tank wire touch some B+ or some with voltage and that is what caused this whole mess. C9 is one of those tropical fish caps and has a huge crack right down it's length, perhaps more than leaky.

Anyhow I don't like the look of R218 it looks real stressed (refer to pic) and it measure about 280-300k. It is supposed to be 180k and all I could find was 1/2 watt 200k comp. resistor for a replacement. I also got a 1 watt 200k too. Should I stick with 1/2 watt or go with the 1 watt for the replacement? 1/2 watt says 350v working voltage and 1 watt says 600v, so I am leaning towards the 1 watt. As should be, these amps make you a bit nervous the closer you get towards firing it up! lol. As always thanks for any help.

[The Dude]

That resistor looks to be a dropping resistor for a standby lamp near as I can tell. (Schematic is a bit blurry when I expand it). Half watt should be fine, but either would work. I don't see it as a big deal.

[DrGonz78]

So premlinary checks are good. No significant current draw and the bias supply looks good to go. I do have one more question about the bias supply between V4 models. On this particular amp it states -28.5v and on older models it is around -62v for bias supply. I do see differences in the circuits as far as the newer ones use extra secondary winding for screen and bias circuit power supplies. Still curious as to why my model is -28.5v and other older ones are -62v. Thanks.

[g1]

The big difference in the bias voltage is due to the radical difference in the screen voltage.

[The Dude]

You sort of answered the question in your post, Dr. The difference in bias is because of the difference in screen voltage and where it comes from.

Edit: Ahhhh, you beat me to it g-one.

[DrGonz78]

Okay I see now that I was missing the screen voltage differences between versions and now it makes way more sense. Yeah I even knew the screens voltages were different but man oh man that's huge!!

[DrGonz78]

So amp is passing signal and sounds great! B+ voltage is coming in at 532v and the bias supply is at -32v. So -28.5v on the schematic would be a touch hotter bias than what I have right? Channel 2 is not really working still and got to trouble shoot that now. Using my variac the amp sits at 2 amps at idle on the ammeter guage does that also seem about right for this beast?

[The Dude]

Glad you got it up and running! Yep, the more negative the bias, the colder. This looks to be fixed bias, so just make sure you don't see any red plating. If you have a bias probe, it wouldn't hurt to check the actual bias current, but probably not a necessary thing. I check every amp with the probe just to be sure. On to channel 2 you go.

[DrGonz78]

I have a quad style bias probe set and I will check current draw of each tube too. Yup no red plating and so far nothing out of the ordinary. Yup channel 2 could just be a really dirty volume pot as some guitar signal came through(in bursts then stopped) but just really really scratchy as you turned it. All the pots need some loving care.

[g1]

Those things are prone to oxidized sockets and also solder on socket issues, wiggling preamp tubes will reveal that.
I'm curious to see how the idle current looks for the power tubes. The plate voltage seems a bit low, which could mean hot bias, but the probe will tell.

[DrGonz78]

Yeah I am right there thinking the same thing now G-One, good call. I thought about my B+ being a bit low and thus higher bias as a result. The bias probe readings indicate a higher bias than I would want. I had posted my B+ at 532v and now testing with bias plate probe I get 521v at the plate. That makes sense since the B+ drops across those 5watt resistors at the plate.

30/521 = .057ma and .057 * 0.5 = 0.0285(28.5ma). >>> So I should read 50% max dissipation at 28.5ma on the probe. One set of tubes is reading about 56.5ma and the other two are at 58ma.

Well now time to figure out why the bias is twice as much as it should be... The plates are definitely not red plating and that is the good part. However, there is no way I can call the repair final at this point. I will talk with the amp owner and let him know there are still further repair issues to conquer on this one.

I guess I need some good advice on how to fix this bias problem. Please help, thanks.

[DrGonz78]

Could the problem not be the bias supply and really just low B+? Since the bias supply is intact I am begining to think that I need to check some of the dropping resistors in the power supply. Does that seem feasible? I will also have to give some more test point voltagess too as I go further.

[g1]

Your power tubes are idling at about 100% so that explains the plate voltage. As you make the bias colder, the plate voltage will come back up to where it should be. It takes a lot more than 100% to red-plate, so just because tubes are not red does not mean bias is not hot.
What are your voltages at pins 4 and 5 of power tubes?