Selmer PA/100 restoration

[janos.hardi]

Hey guys,

I'm in the middle of restoring a Selmer PA/100, I believe a mark 2. Schem here: Selmer PA 100 MkII Schematic
It is in a terrible shape as it toured for decades constantly and was neglected, butchered but it looks like the main parts are intact. I'm not planning to change the circuit into yet an other marshall etc. but give it a decent and safe power supply circuit + general cleaning. I really like the unassuming look.

Now to the question.

On the schem and in the circuit there is an EF86 pentode that receives the four preamp signals (and in some of the amps an echo in, the version I have does not have the echo aka send/return) and drives the phase inverter. I'm guessing it used to be a textbook example in the 60s amps but I never saw anything like it. Selmer used this in other amps but in the mid 70s they changed this position to a "standard" 12AX7. Do you know other examples where this has been done? And for a good reason?

Thanks!

 

[trobbins]

Some comments that may be relevant are:
- the circuit aims to reduce the gain of the pentode quite a bit, as the 2k2 cathode is un-bypassed, so maybe no substantial stage gain difference than a 12AX7 triode would achieve.
- the EF86 needs more parts (ie. screen supply) and although designed to be less microphonic, its field performance over time may have shown up some microphonic examples.
- latter amp models may have been able to utilise the 2 triodes in a 12AX7, and so gain some cost benefit.
- as the stage gain and high frequency roll-off may be different if a 12AX7 is used, the feedback design may need to be tweaked (not just for basic dB level) if you are contemplating altering the valve being used.
- the stage is dc coupled to the PI stage (to improve feedback stability) and so the EF86 anode idle voltage of 90V needs to be in a practical range (that voltage may drift over time and cause issues, and would also need to be managed for any change to a 12AX7 triode).
Ciao, Tim

 

[janos.hardi]

Thanks Tim,

Yes, all relevant comments! I came to the somewhat obvious conclusion that the choice of pentode here is (probably) because they were widely available. Been looking into other 60ies (pa) amplifiers and the variety of valves being used is ashtonishing. ECC83s as preamps were widespread but then mixers, phase inverters and drivers (not to mention the power tubes) - they came in variety. I have a small swiss made pa amplifier with ECF80 PI and EL500 power tubes. Sunn also used a pentode-triode in almost all in their amps so my guess is they had a truckload of them and so it became part of the design.

I might change it to ECC83 since I have to update a lot of components anyway.

Thanks!

 

[trobbins]

Are you wanting to change from EF86 to ECC83 on a whim, or is there a technical or practical reason?

 

[janos.hardi]

Are you wanting to change from EF86 to ECC83 on a whim, or is there a technical or practical reason?

Honestly I might change my mind and just keep the EF86 It will be a long way to go to redo the circuit. I have plenty of old Tungsram EF86s and ECC83s. The later SV circuit uses an ECC83 in that position without major changes in the circuit elsewhere. Not sure yet. My main concern is the state of the transformers and the choke. Continuity seems to be okay but I have no means to test them properly (yet). After finishing up my Hiwatt tribute head I have more time to focus on this big guy.