Selmer Amp: Too much gain!

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Selmer Amp: Too much gain!

Postby juniormarbles on Mon May 15, 2017 5:01 pm

I recently got a Selmer Truvoice Selectortone Automatic amp (2xECC83, 2x EF86, 2X EL34, GZ34) that is mounted in a Croc Zodiac 30 2x12 cabinet w/two 8Ω Celestions wired in series.
I love everything about it, except for the amp's stupendous gain that saturates into ugly ("farty") distortion anywhere past 1/2 on the volume knobs in both channels. All tubes and passive components checked out excellent, all supply voltages measured are healthy, etc. etc.

To lower the gain I tried 12AY7 instead of 12AX7 in V1 (first preamp tube) and V4 (phase inverter/mixdown). But that barely lowered the gain. Then I lowered the plate resistor of V1 from 220k to 100k, again, barely noticeable.

What is the trick to get this amp to respond a bit less distorted at higher volumes, and to rid the distortion of its ugly edge, so I can get the power tubes engaged a bit more in the tone formation, rather than being flooded with preamp distortion?

Any hints from Selmer aficionados and techs will be appreciated.
JM
Last edited by juniormarbles on Fri Jul 28, 2017 8:01 pm, edited 1 time in total.
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Re: Selmer Amp: Too much gain!

Postby Vortexion on Mon May 15, 2017 9:26 pm

Hi JM,

There seem to be at least a couple of conflicting schematics floating about the web, so forgive me if this info doesn't apply in your case, but...

If the ECC83 double triode used for the first stage of Channels 1 and 2 (i.e. V1a/V1b) is fitted with cathode bypass capacitors, you could try removing them. That would cause a serious drop in gain of that stage and make the valve cleaner/more linear.

I say "If..." because so far I've seen one schem that shows no bypass caps ("Selectortone Twin") and another that shows them in place ("Selectortone Automatic"). On this latter schematic, the caps I'm talking about are C1 and C10, which are listed as 25uF electrolytics.

Hope that might help.
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Re: Selmer Amp: Too much gain!

Postby stratele52 on Mon May 15, 2017 11:03 pm

Look at this schematic, is it your amp. ?

http://www.vintagehofner.co.uk/selmer/s ... tosel.html
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Re: Selmer Amp: Too much gain!

Postby juniormarbles on Tue May 16, 2017 12:27 am

Yes, the schematic represents my amp exactly, with all components and values present here. (Though there's a drawing mistake on V3 of channel 2, it does not affect my issue.)

Would removing the bypass cap of V1 bring me salvation? Nothing easier tried that this.
Thanks,
JM
Last edited by juniormarbles on Fri Jul 28, 2017 8:04 pm, edited 2 times in total.
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Re: Selmer Amp: Too much gain!

Postby stratele52 on Tue May 16, 2017 8:36 am

You write ;
".... distortion anywhere past 1/3 on the volume knobs in both channels. All tubes and passive components checked out excellent, all supply voltages are ...."

1- Any mod on this amp ?
2- Did the amp work fin before or you receive it with distortion ?

How do yo check tubes ? Tubes tester are not reliable for that. You must put know good tubes.
Working on channel one ( C1 ) will not fix channel 2 distortion. Your issue is on common stages use for both channels

Common stage are ;
Phase Inverter circuit V4b
Output tubes V5, V6
Output transformer.

Did you check output transformer ? It may be the issue if you are sure every thing is good
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Re: Selmer Amp: Too much gain!

Postby trobbins on Tue May 16, 2017 9:32 am

Are you using a high output pup? Are you using the #2 input jack, that attenuates input level?

Are you able to push the PI/output stage just in to overdrive by using a lower input signal, and winding volume up to max?
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Re: Selmer Amp: Too much gain!

Postby pdf64 on Tue May 16, 2017 9:43 am

The parts list describes the volume controls as 'semi-log'; maybe that taper is too steep, or perhaps linear types have been used, either as stock or as replacements.
If so, 10% audio taper pots, eg regular CTS, for the volume controls may fix the issue.
To identify the taper, set the control to 50% rotation, isolate the hot and wiper terminals and measure the resistances between 0V, wiper and hot terminals.
Last edited by pdf64 on Tue May 16, 2017 3:31 pm, edited 1 time in total.
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Re: Selmer Amp: Too much gain!

Postby stratele52 on Tue May 16, 2017 11:21 am

pdf64 wrote:The parts list describes the volume controls as 'semi-log'; maybe that taper is too steep, or perhaps linear types have been used, either as stock or as replacements.
If so, 10% audio taper pots, eg regular CTS, for the volume controls may fix the issue.
To identify the taper, isolate the hot and wiper terminals and measure the resistances between 0V, wiper and hot terminals.



Yes, if it is almost full volume at 3.
If not, pot may be the good one.
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Re: Selmer Amp: Too much gain!

Postby pdf64 on Wed May 17, 2017 5:15 pm

Given that schematic, I can't think of any likely failure modes that would increase gain significantly (without also crippling the power output)?
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Re: Selmer Amp: Too much gain!

Postby Mr. Foxen on Wed May 17, 2017 11:07 pm

Gain and distortion are different things. What you have is a distortion problem, it is not inherent to those amps, so something is wrong with it.
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Re: Selmer Amp: Too much gain!

Postby dread_lion on Tue Jul 25, 2017 10:18 am

i had the exact same problem on 2 amps already. all components had been tested, all voltages looked correct, but i could barely get any clean out of them. they distorted like mad. turned out to be a partially shorted output transformer. it still worked, but just not like it should. so maybe its not a bad idea to check with another transformer if you have one.
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Re: Selmer Amp: Too much gain!

Postby juniormarbles on Fri Jul 28, 2017 5:20 am

stratele52 wrote:1- Any mod on this amp ?

None, until I tried to address the high gain/ugly distortion issue by lowering pre-amp gain (12AY7, increased R9, R19, etc.) all of which did not do much.
I also reduced the coupling capacitors of all stages to tame the bassiness a bit by replacing them with .0068 Mullard mustards. That was successful. The bass heaviness is gone.


2- Did the amp work fine before or you receive it with distortion ?

Received it with distortion.


How do yo check tubes ? Tubes tester are not reliable for that. You must put known good tubes.

I replaced all tubes, and several times, with known new, good ones. That includes the EL34 power tubes whose current and dissipation was within specs and close, tube to tube.
Your issue is on common stages used for both channels

Common stage are ;
Phase Inverter circuit V4b
Output tubes V5, V6
Output transformer.


Power tubes and V4b phase inverter half of ECC83 checked good.

Did you check output transformer ? It may be the issue if you are sure every thing is good

I had the exact same problem on 2 amps already. All components had been tested, all voltages looked correct, but I could barely get any clean out of them. they distorted like mad. Turned out to be a partially shorted output transformer. it still worked, but just not like it should.


So, is the OUTPUT TRANSFORMER the culprit? How would I check that? The amp certainly is loud, though the "farty"-type ugly distortion, especially at higher levels, is a complete turn-off. I have never experienced anything like it in any amp, small or big. Any ideas how I can analyze whether this unique output transformer is defective? I unfortunately don't have a warehouse full of Truvoice 2 x EL34 into 16Ω transformers...?
Last edited by juniormarbles on Fri Jul 28, 2017 8:17 pm, edited 2 times in total.
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Re: Selmer Amp: Too much gain!

Postby Gooch on Fri Jul 28, 2017 10:20 am

OK, if it's still loud and you have nasty farty distortion, I'd check all coupling caps for DC leakage. Also check cathode caps for good measure. AND, make sure the problem is not with your speaker.
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Re: Selmer Amp: Too much gain!

Postby juniormarbles on Fri Jul 28, 2017 7:57 pm

I will do as you and a previous post suggested: check the mix-down stage, and check all coupling and cathode caps.
I will report back.

Any more ideas how I can verify that the OT is OK?
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Re: Selmer Amp: Too much gain!

Postby juniormarbles on Fri Jul 28, 2017 8:05 pm

pdf64 wrote:The parts list describes the volume controls as 'semi-log'; maybe that taper is too steep, or perhaps linear types have been used, either as stock or as replacements.
10% audio taper pots, eg regular CTS, for the volume controls may fix the issue.
To identify the taper, set the control to 50% rotation, isolate the hot and wiper terminals and measure the resistances between 0V, wiper and hot terminals.

I will measure it. But the ugly quality of the distortion* hints at a problem larger than the volume pot taper.

*If I could describe it: the amp goes from clean to instant midrange fuzz-fart, without the "creamy" type of 3rd harmonic onset so desirable in tube amps ever present. In other words, I can not control or navigate between clean and light distortion by adjusting either my picking force or the guitar's volume pots. The nasty distortion is the same whether using a single-coil Strat or Humbucker Les Paul.

By the way, the amp is equipped with two 12" 8Ω Celestion Greenbacks wired in series, whose sound signature I am very familiar with from using them in Fender and Marshall amps
Last edited by juniormarbles on Fri Jul 28, 2017 8:19 pm, edited 1 time in total.
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