Biasing a '64 Vibroverb Custom

[flatfive]

Anyone have one of these? I can't figure out how it's supposed to be biased. The schematic says 38mA, but there is a SS recto and a GZ34. Schematic doesn't say which one and there is like an 8mA difference between the two.64 Vibroverb Custom Schematic.pdf

[Enzo]

First, bias is nowhere near as critical an adjustment as everyone seems to think. 38ma is a ballpark figure. Since you have an 8ma range, split the difference, 38ma is the target, so 34ma one way and 42ma the other. That should put 38ma in the middle.

The rectifier tube will drop more voltage than the diodes alone, so the B+ will be lower. That will affect current.

At 455v on the plate, 38ma is 17.3 watts dissipation, not particularly hot, and no stress on the tubes. 42ma would mean 19 watts, still no stress.

[flatfive]

Thanks. Actually the SS is the higher current.

[MWJB]

As it will be on the schem, see Enzo's note: "The rectifier tube will drop more voltage than the diodes alone, so the B+ will be lower. That will affect current."...hence the SS rectifier will alway show more mA for a given bias setting.

[flatfive]

Yes, I see that.

[g1]

Schematic shows rectifier switch in "tube" position. Power output test specifies switch should be in tube position. I would think power supply voltages and bias settings would be with switch in tube position also.

[flatfive]

That's what I thought although it doesn't actually say that.

[Enzo]

But it does say that. Note 4 specifies test conditions, one of which is to set the power supply to tube. That means all measurements, including bias, etc.

[MWJB]

No it doesn't, it says "TP27-30 measured with an oscilloscope - set to tube" indicating AC voltages, not dc bias readings. If bias is set to 38mA with a GZ34 then selecting the SS rectifier won't cause instant meltdown when the bias rises by a handful of mA, but as they stipulate 33-38mA I would assume - even though I shouldn't ;-) - that 38mA would be at the highest B+ scenario.

Edit: TP27-29 are actually dc voltages read at the trem, but are to be read under signal test, as opposed to idle dc - B+/bias, etc, read with no signal.

[Alex R]

eight thousandths of an amp ain't a whole lot

[Enzo]

I guess I don't understand what the disagreement is. AS I read it, all tests are made with normally open rectifier switch set for tube. The trem levels, as opposed to the rest of the amp, are set using an oscilloscope rather than meter, since they are a waveform. But still done with B+ set as tube.

[MWJB]

There's no disagreement as such, just no specific stipulation in the notes as to whether Tube or SS rectification is required for the dc idle test...given that the difference between the 2 modes is a handful of volts & mA on the plates, it's not life or death. Real world voltages, once you have the amp plugged in to wall AC, possibly in another country/continent, will vary more than readings however the rectifier switch is set (I know you already know all this Enzo)...but I'd suggest that you are unlikely to need/want much over 40mA per tube at the highest B+ scenario (especially as actual plate voltage may well be higher than that shown). The tubes supplied with the amp are pretty sturdy, but some current production type will only take low to mid 20's plate dissipation, less margin for current to rise in fixed bias.

[MWJB]

I guess if I'm saying anything, it's take all figures on schems with a pinch of salt & use good judgement, because no matter how detaild the schem, there is no guarantee that any amp is going to match it volt for volt.

I'm sure I could have done that with a lot less effort, in hindsight! :-)

[g1]

Just to further nit pick, the discrepancy is due to the poor editing of the notes on the schematic, the note for tp27-30 actually reads bottom to top as follows:
"TP27-30 measured with an oscilliscope, with no input and vibrato switched on"
The "rectifier switch set to tube" statement just kind of ends up hanging there, I would think it was originally tied to the "all DC voltages measured with no input signal" but that is speculation.
I only caught this because read from top to bottom, the tp27 note would say "tp27-30 measured with an oscilliscope, with NO rectifier switch set to tube".

[flatfive]

A very confusing piece of writing.