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Post subject: Twin Reverb Ultra Linear Silverface Ch1 Gain Low
Posted: Thu Oct 25, 2012 6:28 pm
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I have a 74 (my friend thinks) 135w Ultra Linear Twin Reverb Silverface on my bench at the moment.

The gain on input 1 (non reverb channel) is much lower than that of input 2. Tube substitution confirms it's not the 12AX7s etc. Bias to both 12AX7s is correct, and related components are in tolerance.

The amp is recapped, and all voltages test as per schematic. I've checked all passive components to make sure that they are the correct value, although I am yet to lift any of them off the board to be 100%.

Am I missing something here? Is input 1 supposed to be lower than input 2? Test where also performed with the reverb and footswitch NOT connected (mainly due to cord length while on the bench).

I've poured over the schematic for a good while, and cannot fathom why.

Also, does anyone know which layout diagram this amp should use? AB763, AA769, or AA270. I have the correct master schematic, but would like to be sure on the layout diagram.

If I need to post more info or photos just shout up.

Thanks for any help.


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Post subject: Re: Twin Reverb Ultra Linear Silverface Ch1 Gain Low
Posted: Thu Oct 25, 2012 9:32 pm
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Is this the schematic you are looking at?

http://ampwares.com/schematics/twin_reverb_sf_135.pdf

"Input 1" and "Input 2" usually refer to the high and low level inputs of each channel, "Input 2" being 6 dB lower than "Input 1".

Do you instead mean to say that the non-reverb channel has less gain then the reverb channel? How low is "much lower"?

I see V4B as part of the mixer circuit for the reverb and preamp. Even with the reverb switched off, it is probably adding some gain in the channel.

I have not seen a layout diagram for this amp and it certainly wouldn't be any of the ones you listed since they are not ultralinear amps.

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Post subject: Re: Twin Reverb Ultra Linear Silverface Ch1 Gain Low
Posted: Fri Oct 26, 2012 4:04 am
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bluesky636 wrote:
Is this the schematic you are looking at?

http://ampwares.com/schematics/twin_reverb_sf_135.pdf

"Input 1" and "Input 2" usually refer to the high and low level inputs of each channel, "Input 2" being 6 dB lower than "Input 1".

Do you instead mean to say that the non-reverb channel has less gain then the reverb channel? How low is "much lower"?

I see V4B as part of the mixer circuit for the reverb and preamp. Even with the reverb switched off, it is probably adding some gain in the channel.

I have not seen a layout diagram for this amp and it certainly wouldn't be any of the ones you listed since they are not ultralinear amps.


Thanks for the reply. Yes I am using that schematic. What I lack is the component layout diagram to help me check (more quickly) what has been changed by another tech. The AA270 is close and seems to match at least the correct values and layout so far as the main tag board is concerned.

The non reverb channel has less gain than the reverb channel. I do not have a db meter so can't be accurate, but if we set master vol at 10, non reverb channel to 8 this gives us the same 'loudness' as channel two set at 4. Input source was constant from signal generator. This was also replicated from the owners Gibson.

Looking at the schematic again, it seems the input stages V1 and V2 are almost identical, apart from the output from V1b is attenuated by a 220K resistor, and delivers about 1700mv into the next stage, whereas at V2b has an output of 2000mva 15% increase. By the time we get through V4b we have 4000mv, so I surmize that input 2 (reverb channel) has a lot more gain that input 1 by design?

Please tell me if I am completely wrong on this :)


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Post subject: Re: Twin Reverb Ultra Linear Silverface Ch1 Gain Low
Posted: Fri Oct 26, 2012 6:58 am
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I have just been working on my 76 Silverface with a Reverb issue, now resolved, during my fault finding I did notice there is a gain difference between the Non effects channel and the effects channel. Not as much as you describe, I would say if the effects channel was on 4, the non effects channel would have to be on about 6 to get the same volume. It's always been like this, there is a gain difference between the two independent channels, eventho the scheme looks to be almost identical at least in the early stages.
Can't comment if yours is much greater than mine, but there is a noticable difference if this helps.

Best of luck


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Post subject: Re: Twin Reverb Ultra Linear Silverface Ch1 Gain Low
Posted: Fri Oct 26, 2012 9:04 am
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nodric, when you said the caps were all changed, did you mean both electrolytic and non-polar coupling caps? This sounds like maybe a coupling cap issue, if all other test points are ok.

Did the amp have this problem before you put it onto the bench? I've found some new coupling caps (esp "boutique" ones) --- can leak DC, from the get go. You can test for DC leakage across the cap (anode side versus grid side).

Check the resistor network on the input jacks. May have a bad solder point or resistor(s). And the shielded wires from the jacks to the input stage grids.

Good luck!


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Post subject: Re: Twin Reverb Ultra Linear Silverface Ch1 Gain Low
Posted: Fri Oct 26, 2012 10:42 am
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BMW2002Ti wrote:
nodric, when you said the caps were all changed, did you mean both electrolytic and non-polar coupling caps? This sounds like maybe a coupling cap issue, if all other test points are ok.

Did the amp have this problem before you put it onto the bench? I've found some new coupling caps (esp "boutique" ones) --- can leak DC, from the get go. You can test for DC leakage across the cap (anode side versus grid side).

Check the resistor network on the input jacks. May have a bad solder point or resistor(s). And the shielded wires from the jacks to the input stage grids.

Good luck!


Checked and cleaned all the pots. Check all the coupling caps and related resistors, and all OK. Coupling caps have been done, albeit with no name brands. I have an old Heathkit cap leakage tester :)

Tested and now the delta between the channels is closer, but still a good difference, but not so large as I first thought. Channel 1 is also whisper quiet with no signal, even at max vol. I guess it was meant as a practice channel, or small venue/home use?

Anyway, it all works as intended. The original problem was a static rumble when playing a strong note D. Never got to the why it was, but socket & pot cleaning and bias adjustments seem to have made it go away.


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Post subject: Re: Twin Reverb Ultra Linear Silverface Ch1 Gain Low
Posted: Fri Oct 26, 2012 10:53 am
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nodric wrote:
so I surmize that input 2 (reverb channel) has a lot more gain that input 1 by design?


Yes, it would appear that the vibrato/reverb channel has more gain than the normal channel.

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