New problem with my old DR style build.

[Blackdog]

Hello guys,

I have been playing my old build (dual 6V6 Deluxe Reverb style) against the new one (a blackface Vibroverb clone) and I noticed a new behavior in the old build. It distorted rather evenly pretty much at any volume. Not a lot, but clearly distorting.

Put the sine generator and the resistive load and connected the scope: it has a very significant crossover distortion. I tweaked the bias until the distortion essentially disappeared, but this was at the end of the range of the bias pot: 55mA per tube. Of course I have a bit of red-plating at this dissipation, but the distortion fully disappears and the amp sounds great again: i.e. breaking up only when it should, in pure DR style.

I know it didn't have this problem before, as I biased the amp at around 25mA and measured the output power with the scope about 2 years ago. I haven't played the amp that many hours... The only change since then is that I replaced the power transformer (it had the 110V part used in the Fender DRRI, it now has a 220V replacement for the "real" BFDR tranny) and all the voltages went significantly up: I had around 390V +B with the old tranny and now I have around 430V.

I will not have the time to put it back on the bench till maybe tomorrow, so for now its just guessing. But what do you think that could have happened ??

Many TIA.

[phsyconoodler]

Put your scope away and concentrate on bias voltages,plate voltages and current.NEVER use a scope to find out where the distortion stops.You found out,or at least THINK you found out.
  Two things come to mind here.
1)there should be no difference between the DRRI PT and the 'real' one you have.They both should be the same.
2)you have one tube that's shot and that's why it distorts.

  Look at the original DR schematic.It shows about 415 volts on the plates.The only thing that can raise the voltages is if one tube is not drawing current or the rectifier tube is a modern one vs the old stock 5AR4.
  What about wall voltage?
Add 20% to the voltages and 430 seems ballpark.390v is really quite low for an AB763 Deluxe Reverb.
 

[Blackdog]

Thanks for the reply !!
A few additional comments:

1) The transformers are indeed quite different. The new one is significantly bigger and heavier. I had to modify the mounting to the chassis a bit to make it fit.
2) this could be, I'll try.

I have 1 ohm resistors on the 6V6's cathodes. Both tubes are drawing 53-56 mA each.
Rectifier is a 5AR4.
Wall voltage is 220V. The new transformer has 220, 230 and 240V primary. I'm using the 240V tap right now, with the 220V tap I had 460V +B !!

[phsyconoodler]

53ma is too much for 6v6's.They will melt.Try 30ma tops.
   What is the heater voltages with the 240v tap?You may have to use a Zener diode(s) on the center tap to get the voltages in spec.

[PRR]

> what do you think that could have happened ??

1) Lost your NFB.

2) Changed your load impedance.

3) That voltage IS "too high"; though the old 390 was too high for pure-clean, and I would not expect 10% to be a real difference.

Put 6L6 in it, bias to 60mA each.

[Blackdog]

53ma is too much for 6v6's.They will melt.Try 30ma tops.
   What is the heater voltages with the 240v tap?You may have to use a Zener diode(s) on the center tap to get the voltages in spec.

Using the 240V tap and with all tubes in place (2x6V6, 6x12Ax7) the heater voltage is still a healthy 6.1V. The 5AR4 heater voltage is 5.1V.

The 6V6s I'm using are NOS GE, which I bought when I built the amp. They were a bit unbalanced since day one, but the distortion was not there before with a more normal bias. Maybe it is not related to the transformer change at all, could it be the normal degradation of the tubes ??

> what do you think that could have happened ??

1) Lost your NFB.

2) Changed your load impedance.

3) That voltage IS "too high"; though the old 390 was too high for pure-clean, and I would not expect 10% to be a real difference.

Put 6L6 in it, bias to 60mA each.

1) I will check. I have a 5Kpot in series with the 820R in the NFB loop (that I usually keep at minimum, NFB = 820R), but last time I checked it was working fine.
2) It's a combo, same 8ohm Celestion V30 speaker always. The OT doesn't even have any other taps.
3) I guess you mean that 390V is too LOW for pure-clean, right. The schematic shows 415V, do you think the current 420V is too high (as in "have to do something about it") ??

I'd rather keep it with 6V6s, I like the breaking-up character of the DR. I have a set of Fender branded 6L6s pretty much unused in the parts drawer, they sounded a bit harsh, but I could use those to test if there is something with the tubes.

Tonight I may have some time for testing...
Thanks for the replies !!!

[PRR]

> I guess you mean that 390V is too LOW

No.

At 300V and 8K load, a couple 6V6 can be run with idle current the same as max-power current. Crossover distortion is nil. Or to be more precise: idle 2*40mA, peak 120mA. Gain is about the same at both extremes.

At 400V, you must reduce idle current so the tubes don't melt. However the high V sucks a high peak max-power current. Now you are near 2*30mA idle and 180mA peak. Gain near the peak is significantly higher than gain near idle point.

However over-volting tubes and mild crossover is traditional for guitar. So I wonder what has changed.