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FDP Forum / Amp Mods, Repairs, and Projects / Showman reverb weak tremelo

swampyankee
Contributing Member
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olde New England

If you can't play good, play loud
Apr 26th, 2017 07:56 PM   Edit   Profile   Print Topic   Search Topic

After a couple tube replacements, my new 1970 DSR up and running, and I'm digging the tone, the reverb,...the lack of tremelo...

At first I thought it was non-existent, but if I listened close with depth on 10, I can just make it out. What are my first steps toward troubleshooting my weak tremelo?

(This message was last edited by swampyankee at 09:58 PM, Apr 26th, 2017)

amphead4

Cincinnati, USA

Apr 27th, 2017 08:35 AM   Edit   Profile   Print Topic   Search Topic

First step is sub the tube.

Next is to measure the voltages.

Try to determine if the problem is on the oscillation side or the LDR driver side.

Since it is oscillating though, focus on the drive side, V5B. It needs enough gain to drive the neon light so make sure the cathode voltage is per the schematic. You can jumper in another cathode bypass cap in parallel with the first one to see if that helps. If so, replace that cap.

If the driver triode seems to be working fine, maybe it's the LDR unit. Do you see the neon light firing?

Ohm out the intensity pot to make sure nothing is flaky there.

swampyankee
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olde New England

If you can't play good, play loud
Apr 27th, 2017 03:12 PM   Edit   Profile   Print Topic   Search Topic

I have the chassis out, so I swapped tubes, going by the explanations and recommendations on the T.A.D. site. I made sure I had the best ones recommended for the tremelo tubes. I can see pulsing light coming out of the ends of the shrink tube around the opti-bug, so that's working. Now with tremelo on and intensity at 10, I get a bit more tremelo action, albeit nowhere near my Vibroverb, which practically goes from 0 to 10 in volume when the intensity is that high.

(This message was last edited by swampyankee at 06:06 PM, Apr 27th, 2017)

Hammond101
Contributing Member
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So. Cal. USA

Apr 27th, 2017 04:31 PM   Edit   Profile   Print Topic   Search Topic

Sounds like the resistor in the LDR (trem bug) has given up. Did you check any voltages?

swampyankee
Contributing Member
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olde New England

If you can't play good, play loud
Apr 28th, 2017 07:54 AM   Edit   Profile   Print Topic   Search Topic

Not yet. I'm hoping to get some quality time with the amp this weekend. I figure to check the intensity pot as well as the bug resistor. Funny though, that a swap of tubes into the tremelo controls improved the max tremelo intensity from about a 2 to maybe a 5

amphead4

Cincinnati, USA

Apr 28th, 2017 08:26 AM   Edit   Profile   Print Topic   Search Topic

swampyankee, it's never going to have tremolo that sounds like your Vibroverb RI because the circuit is different.

The Vibroverb trem circuit modulates the bias of the output tubes. This is like running a constant signal into the power tubes from the phase inverter and adjusting their gain up and down.

The Showman uses the trem bug is which is like an electronic control over a volume pot, turning the signal down and back up. Although the oscillator may put out a smooth sine-like waveform, the bug is not at all linear so it can't respond smoothly to that input. And the bugs themselves vary somewhat in performance from one to another.

swampyankee
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olde New England

If you can't play good, play loud
Apr 28th, 2017 11:25 AM   Edit   Profile   Print Topic   Search Topic

I understand the silverface tremelo is not as intense as the Vibroverb. The tremelo on my Vibroverb is ridiculous. Intensity at 6 or 7 is more than I'll ever need. 3 or 4 gives a nice, smooth effect. Right now, Showman on 10 is equivalent to the Vibroverb on 2 or 3. Not something that would come through in a band mix.

Hammond101
Contributing Member
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So. Cal. USA

Apr 28th, 2017 11:32 AM   Edit   Profile   Print Topic   Search Topic

My throw parts at it repair is typically to swap the bug and change the ceramic disc caps in the circuit once a good tube is confirmed. Very rarely does this not solve the ills other than the "tick" cap mod that sometimes needs to be added or changing the middle cap to a .02 to slow the trem.

A lack of dwell, the amount of intensity typically is a weak bug.

RDR
Contributing Member
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I tried to think

but nothing happened!
Jun 17th, 2017 04:20 PM   Edit   Profile   Print Topic   Search Topic

How timely for me, thanks for all the info. My 66 BM wobble is pretty weak all of a sudden.

FDP Forum / Amp Mods, Repairs, and Projects / Showman reverb weak tremelo




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