Tube Amp Archive

Same in both channels?
With the exception of the overall bass level, do all controls still work as normal?
Any other changes go along with the reduction in bass?
A voltage survey in normal and fault modes may be beneficial.

Mark wrote: ↑Sat Apr 23, 2022 9:12 am ... The earth layout for the preamp side is a little sketchy..

Yes, I don’t think it’s an example of best practice. I get the impression that there’s a Fender style brass or copper plate behind the control panel that may be seen as a low impedance common star point for all the preamp.

... the bias current doesn't seem to stop rising. It's very slow, but I haven't found the top point and I'm not sure that it's related to this distortion. Only thing I can think of at the moment. How about if the amp is operated with the valve in V3 is removed? ie does the output valve anode or cat...

There’s 3 yba3 schematics in that manual. The 1st has the regular 5F6A type presence control, 2nd and 3rd don’t. 1st is hand noted as being the 1st model, 2nd looks to be introduced Jan 13 1967, 3rd is the same basic drawing as the 2nd, but starts from revision 10, Nov 1970. The amp of this thread i...

Note that if a typical 10 to 15% audio taper pot has been used for the treble control, it may be beneficial to try setting it higher than you’d expect if used to a real Fender, as they typically used a treble pot with a taper of about 30%.

The TR AB763 had a 1nF cap at the power amp input, for some models intended for closed back cabs it was even lower, eg 500pF.

The layout for the Weber kit looks a bit more logical https://www.tedweber.com/6v30-c-kt/ https://tedweber.com/media/kits/6v30_layout.jpg So that might help with devising a better 0V common arrangement. I use pull push pots for both vib/trem effect control panel switch positions. Speed becomes a var...

That plate voltage sounds right for a 1987x. Check your bias voltage on pin 5 of both EL34s, then check it where the 220K grid leak resistors meet. It should be the same in all three points. Just to note that it can’t be exactly the same though, unless a vtvm is used. The 10M loading of a typical D...

For sure any amps power supply that had enough V+ to make over 35 watts out of a two 6L6 or 6Ca7/ EL34 would need to run JJ brand 6V6’s. Dunno about that, as per the Mullard info, EL34 can put out 50W with the loaded HT at 400V https://frank.pocnet.net/sheets/129/e/EL34_Mullard.pdf My Sound City ca...

... I read somewhere that 6CA7 are somewhat similar tyo 6L6 being a beam forming tetrode. Would that not warrant a different OT than for EL34s My old pair of GE 6CA7 do seem to be a beam pentode. But I think they have been engineered to perform the same as, and seem entirely functionally equivilant...

… I used the Biasing Tube Amplifier Calculator @ https://www.amplifiedparts.com/tech-articles/tube-amplifier-bias-calculator This suggests that for a plate voltage of 446 volts and a 6L6GC tube a current of 47ma will yield a 70% bias. Have I missed something or misunderstood? … I think all these on...

Using the full secondary is not simple if, as is the case with many vintage UK valve guitar amps, there’s a 100V output available (on my Sound City MkIV+ it works out to an intended load of 200ohm). Similarly for 70V outputs. I suspect those windings may be poorly coupled anyway. Poorly coupled / no...

50W Marshalls from the 70s have their silver award for bad standby design. In that standby switches off the bias supply, as well as the HT supply. When the standby switch is closed, the HT comes up virtually instantly, the bias takes a bit longer, subjecting the output valves to a nasty kicking ever...

Just to note that 47mA idle current seems very high for fixed bias AB. Design max limits are intended to be derated, in order to accommodate supply and component variance. When the root cause of the issue is sorted out, cooler idle typically helps to achieve a good trem depth with bias vary type tre...

What source (supply) impedance are you testing it with? eg 50 or 600ohms off the sig gen, something roughly equivalent to the primary impedance, eg 4k7, or something roughly equivalent to the likely anode impedance, eg 22k?
Hammond tend to use a source impedance equal to the primary impedance.