SOUND ON SOUND

[Hugh Robjohns]

I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI.

I think both of those tests show very clearly that the amp is heavily reliant on extremely good earthing to control RF interference. So in situations where there is little RFI around, or where it has a really solid earth, it will probably perform perfectly.

However, whether its susceptibility to RFI is a fault on this particular unit, or a design flaw I can't say. The attentions of a good tech should sort it out relatively easily though, I'd have thought.

H

[ef37a]

I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI.

I think both of those tests show very clearly that the amp is heavily reliant on extremely good earthing to control RF interference. So in situations where there is little RFI around, or where it has a really solid earth, it will probably perform perfectly.

However, whether its susceptibility to RFI is a fault on this particular unit, or a design flaw I can't say. The attentions of a good tech should sort it out relatively easily though, I'd have thought.

H

I am sorry Hugh, I don't think this has anything to do with earthing*. IMHO the extended mains lead is acting as a better aerial and possibly a resonant stub and maximising the RF signal inside the chassis.

*Maybe you are just too young to remember but fitting a good earth to a MF radio always improved the signal!

Anyway, we can debate this problem ad n but the ONLY way to fix RFI issues is to get hands on. OP must take the amp to a tech.

Dave.

[Platypus9]

This is the Combo 1x12 version. I tried the amp from an outlet on the first floor, and the RFI was barely audible. I then ran a heavy-gauge extension cord to the 2nd floor, from the same outlet on the first floor. Result: intense RFI. Given that I have a half-dozen amps, and this is the only one with the problem, I'll be taking this one to a professional.

One small possibility is that you mat be simply moving the amp closer to a localised source of RFI. Anything electronic (probably faulty if it is the case) on the 2nd floor (or the attic) that may be retransmitting radio signals? Baby alarm maybe? Something Bluetooth?

Just turning off all possible sources (including computers) and then testing again might give a culprit if the radio signal vanishes.

Or it may stay the same as before, but it costs nothing to check.

Good idea. I turned everything off, but not surprisingly, it didn't help.

[Platypus9]

Discovery: When I turn the reverb knob to zero, the RFI nearly disappears, and the volume of the RFI is directly proportional to the reverb knob position... I thought the reverb tank was acting as an antenna, but unplugging the reverb tank had no effect on the RFI.

[Hugh Robjohns]

Well, at least you now know which circuit stage is faulty... That will help the technician enormously!

H

[Wonks]

It may be the reverb tank cable that's not helping. Some have a shielded cable and an unshielded cable, so that the tank has only one ground connection. Normally there are RCA phono connections at both ends.

I presume if this is the case with this amp, the shielded cable should be on the output side of the tank, as the signal level is so much lower and then has significant amplification afterwards. So I'd check on this and if so, make sure the shielded cable is on the output. If both input and output cables are shielded, then you can't do much more than make sure that the phono connections are grounding properly. Sometimes it can just be the central signal plug that makes contact and keeps the cable in place, so bending the outer 'ring' leaves inwards can improve ground contact.

V3 is the valve that deals with the reverb return and post-reverb amplification. You could try replacing this with another 12AX7 to see if this improves matters as it's easy to do. Only do it when the valves are cool though. But it's more likely to be a bad ground connection somewhere in this area of the amp.

[Wonks]

I'd also check that the previous owner hasn't changed the reverb tank for another type. It obviously works, but the grounding arrangements within the tank can be different from different suppliers. I've only ever done one reverb tank replacement, but I did notice that the new tank had either isolated inputs or outputs (can't remember which now) whereas the old tank didn't IIRC.

[ef37a]

Maybe we are closing in Wonks?

The '65 schematic shows C11, 220pf inside the tank shunting the output to shield ground. Your comment that a different tank might be fitted might mean the cap is not fitted? (there is also no stopper to V3a grid and since a reverb return needs virtually no HF response past say 10kHz, I find that surprising)

This problem seems to me to be more and more a design shortcoming and lack of pre-release testing.

Dave.

[Wonks]

Difficult to say about stoppers, as fitting them is one of the standard list of recommended Princeton upgrades. So the Morgan PR12 circuit may well have them in, or it may not. But it does look like it's something to do with the return part of the reverb circuit as we know the radio interference level isn't affected by the input volume, just the reverb level control.

[ef37a]

Difficult to say about stoppers, as fitting them is one of the standard list of recommended Princeton upgrades. So the Morgan PR12 circuit may well have them in, or it may not. But it does look like it's something to do with the return part of the reverb circuit as we know the radio interference level isn't affected by the input volume, just the reverb level control.

Slightly OT Wonks and I shall look for those mods soon but the stoppers point interests me.

One reason the concertina PI is not popular is that it is said to not sound good under overdrive? Merlin Blencowe is a great advocate of stoppers and says they fix that problem. I note the OP valve grids have only 1k5, low for even standard designs, 10k is more common but MB suggests 100k or higher. This will not, as many people thnk cause 'tone suck' and rob HF from the OP valves because being pentodes they have diddly grid capacitance.

In any event, 3dB down at 10kHz is easily good enough for gitamps.

Oh to be 60 again with the time and energy to play with this stuff!

Dave.