Peavey AlphaB , what's the deal with ?

[g1]

I don't think Peavey ever said "you need to use Peavey brand tubes." Fender sells tubes with their logo on them too.

That is correct. Peavey never said anything about using their tubes for anything related to bias. That is a mesa thing.

[Enzo]

Peavey sold all the types of tubes their amps used, but they are not graded and are not specified. They also sell guitar straps that say Peavey on them, yet the guitars they make also work with other brand straps.

No peavey tube amp ever came with a warning to only use Peavey tubes.

[catalin gramada]

Contact resistance is in series with the 1R sense resistor, so doesn't change the voltage across the resistor when connected to a current source (tube).
It is essential that the probe leads are directly connected to the ends of the resistor inside the adaptor.

That is a 4-point measuring arrangement and accuracy only depends on the precision of the resistor itself.

Yes, you're right. Errors may come when measuring the voltage relative to ground (as usually do) and not across to resistor terminals as You well pointed. That gave me other ideea. I may put individual resistors in each cathode grouped in one rail per side and each rail with a resistor to ground for easy balance.

[catalin gramada]

This is how I solved with the bias sense resistors making it more controllable on original board.. But still a question please. The amp was not in original condition as the seller said. He sent it to me not with original Sylvania inside but with cheap russian 6P3S. Well is not quite a bad tube as mine are 6P3S-E version , still 23W so the bias was a must to be modified anyway...My concern regard the fact the amp is iddle at 500V DC. I may suppose ( not tested yet) the transformer will slide to more 450 V when throttle...Well how safe this voltages are for 6P3S-E please ? The OT have 1.2k Ra-a, Any thoughts? Thanks.
One screen resistor burnt, the other roasted...it start to be a wrong investment...
At least I wanna see how is sound in good order condition...

[catalin gramada]

It is something confused on the service manual: The heater hum balance circuit consisting by a 250ohm pot. It is show the whiper related to ground by a 100u elco capacitor but : it is show with minus pole to the ground in the schematic and plus pole to the ground on the board drawing ??? How should be mounted right please ? Or better strapped and get rid of it ? Thanks.

[Enzo]

Why get rid of it? And certainly don't short across it. Look at your schematic. The balance pot has the cap to ground from the wiper, yes. Pot is across the heater 6vAC feeds, FILA and FILB. That is on the power tube socket board. Now look at the power supply on the main board. FILA and FILB connect across to the yellow 6v wires from the power transformer. More importantly, look near the center of the FILA line across the page. 4.7k R21 connects FILA to the positive low voltage supply. It isn't regulated so I would estimate 20-25 volts. DC. Therefore, the entire string of heaters sits at about +20vDC with respect to ground. If you short the mystery cap, you will be grounding R21.

So the schematic is correct, the +20v on the balance control requires the + end of the cap.

The layout drawing has the + on the wrong end. It is simply a typographical error.

The hu balance circuit helps reduce hum, and elevating the heaters by 20v goes even further to fight hum. I highly recommend leaving the circuit as designed.

[catalin gramada]

Why get rid of it? And certainly don't short across it. Look at your schematic. The balance pot has the cap to ground from the wiper, yes. Pot is across the heater 6vAC feeds, FILA and FILB. That is on the power tube socket board. Now look at the power supply on the main board. FILA and FILB connect across to the yellow 6v wires from the power transformer. More importantly, look near the center of the FILA line across the page. 4.7k R21 connects FILA to the positive low voltage supply. It isn't regulated so I would estimate 20-25 volts. DC. Therefore, the entire string of heaters sits at about +20vDC with respect to ground. If you short the mystery cap, you will be grounding R21.

So the schematic is correct, the +20v on the balance control requires the + end of the cap.

The layout drawing has the + on the wrong end. It is simply a typographical error.

The hu balance circuit helps reduce hum, and elevating the heaters by 20v goes even further to fight hum. I highly recommend leaving the circuit as designed.

Oh yes, I see. The heater circuit looks quite like that (pics atached). Thanks Enzo !
clear a typo error, those cap have no way to be reversed...

[catalin gramada]

What are the consequences using small current rating plate capacitors ( think 0.5A or so) for the circuit please ? It worth to change it with some bigger cans by same value ?...not to sure the PT is allready undersized too and put it's own current limit, so I.m not sure if have sense to replace it with some bigger current rating cans...may you help please ? We don't wanna keep high ripple curent caps in this position to protect the caps from failure as long as we can ? Thx.

[catalin gramada]

Thinking like that:

[Enzo]

Whenever we want to re-engineer a circuit, I always have to ask this: what shortcoming in performance are we trying to improve? These amps have been around operating for decades, and as a Peavey authorized shop I do not recall them having any consistent complaints. Just same stuff that happens to all tube amps. If the original caps made it 30 years, I am left to believe they were not under excess stress.

[catalin gramada]

Whenever we want to re-engineer a circuit, I always have to ask this: what shortcoming in performance are we trying to improve? These amps have been around operating for decades, and as a Peavey authorized shop I do not recall them having any consistent complaints. Just same stuff that happens to all tube amps. If the original caps made it 30 years, I am left to believe they were not under excess stress.

Hey Enzo, I willl change it anyhow as a good practice service after 30 years work I don't expect to explode. My question regard if it worth to replace it with bigger curent cans I have at hand ? It cost me as same and have to lift the board anyhow to change the rest of caps.Thinking not circuit performance improvements as time I want to preserve same value as circuit was designed, but more reliability and as bonus you have not to disassemble the board next time to change the power cans. I do a general service to this amp, nothing to improve , but found some vulnerable points on it like lacking bias sense resistors, variable bias current circuit and now this power cans, I think it deserve to easy modify to make it more reliable , that's it.

[Enzo]

You expect to be servicing this unit 30 years from now? If you like it, go ahead and change them. I personally see no point in doing so.

[catalin gramada]

Man, this amp was not quite keeped in the shelf, it was abused: screen grids resistors was burned/roasted, the fan cooler is heavy rattling, intermitent operation at volume pots,humming noise...wearing electrolytics is something you expect...like covid without vaccine if you wish...Making bullet proof is a consequence, otherwise this amp was adverted as serviced and functional.(...well, it work..).It well depends what each understand by that...(dust cleaning perhaps..? If that electronics service meant...)

[catalin gramada]

...but I have to agree, looking on data sheets, most modern good quality capacitors this size ar rated nowadays at 0.9-1A and 5000 working hours so no need to hang big cans inside the box but ordering couple of little pricey caps and planted to the board.Is a bass amp, will never have chance to draw as much current to rise the ripple as much to endanger the caps.

[catalin gramada]

An other problem founded at this amp is the power transformer who did not have a proper rated curent ability. I can live with the fact it will sag-otherwise a characteristic of its voicing.But even the filament winding is not usable.For instance. Have 230V on the wall with amp power selector set at 220V .Without power tubes in the voltage on the rail is 6.66V. Then, installing power tubes one by one I get:6.4V with one tube, 6.2V with two tubes...and so on 6.1/5.9/5.6 and finally 5.4V with all power tubes installed. Is clear I cannot use this voltage to supply the heaters and the amp need at least a separate tranny for the heaters. Opinion please ? Thanks.