Put-put noise going when tremolo is engaged

[slidincharlie (Carlo P)]

I have an old Gibson GA18-T that works very well overall, except for a put-put noise (is this motorboating?) occurring when the tremolo is engaged.
The noise starts when the depth knob is around 4-5 on the dial, and is highest when the depth knob is on 10 AND the volume knob is on 1. With the depth knob on 1 there is no put-putting irrespective of the volume knob setting. The put-put frequency follows the speed knob setting.
Could it be a matter of bad lead dress?
I have increased the value of the caps in the tremolo circuit to slow down the speed rate, putting new caps in parallel with the old ones because I didn't have the right values available (see photo below), but the same put-putting occurred also before this job.

I'd appreciate very much your help to troubleshoot this.

[Tom Phillips]

Carlo,
Nice photo. As you probably know the GA-18T has an output tube bias vary tremolo. The thumping sound is often a problem in these amps.
Is that an original multi section cap can in the photo? If so your amp most likely needs a power supply rebuild and that may solve (or reduce) the problem. I see from the schematic that the trem oscillator shares the same power supply node with the power tube screens. If it were my design I would run that oscillator from its own decoupled power supply node.

I'd also verify that the cathode bias resistor is within spec. If the bias is too hot it will contribute to the tremolo put put problem.

I think you may find some additional useful information at http://www.lynx.bc.ca/~jc/tremoloCircuits.html
Look at "Note 3" at the very end of the article.

Regards,
Tom

[slidincharlie (Carlo P)]

Thank you Tom.
I'll work again on the amp hopefully by this weekend.
The multi-cap I suppose it's not original from the late 50s. It's Rubycon brand, and I thought it was relatively recent and not worth to be replaced.
I'll post a picture of it, so we can try to date it.
I have to tell you that the put-putting was present when I bought and played this amp after purchase. Then I played the amp again yesterday and it was quiet. The noise appeared again for some reason after the cap replacement.
I'll check things and post again.

Thanks for the link - I didn't know the JC Maillet's website. It looks a very cool site at a first glance...

[slidincharlie (Carlo P)]

I haven't started to work on the amp again yet.
In the meantime, trying to understand if the filter caps need changing:
how old do you think is this multicap?
The code says 98 08 22 (or: 9 8 08 22).
Is it from 1998?

[Tom Phillips]

I haven't started to work on the amp again yet.
In the meantime, trying to understand if the filter caps need changing:
how old do you think is this multicap?
The code says 98 08 22 (or: 9 8 08 22).
Is it from 1998?

Interesting question. Your asssesment seems to make sense but I thought that Rubycon stopped making can caps before 1998.
It will be interesting to hear what others have to say.
Tom

[slidincharlie (Carlo P)]

Tubes matter!

As you probably know the GA-18T has an output tube bias vary tremolo. The thumping sound is often a problem in these amps. (...snip...) I'd also verify that the cathode bias resistor is within spec. If the bias is too hot it will contribute to the tremolo put put problem.

I have replaced the old, huge, badly leaky .33 Astron cap in the tremolo circuit, and this cured about 60% of the problem. Now there is only put-put (no more toc-toc on top) when I crank the volume and when the intensity knob is above 5.
I have noted that the old RCA 6V6's in there are badly mismatched (25 and 38 mA). Using another two old 6V6's 'almost' matched (26 and 31 mA) almost killed the thumping noise.
Interestingly, two strong Philips JAN NOS 6V6s made things a little worse, causing an increase of thumping (at the same level as the two RCA tubes).
Overall, the different tube pairs I tried were biased around 70-75% max idle dissipation (I assumed 12W as max value).

I'd like to read some opinion and comments on this.

[Tom Phillips]

Carlo,
In your type of amp the bias setting and the max available tremolo are a compromise. I think that 75% of max bias setting is a little too much for an amp with bias vary tremolo. I suggest that you try backing off a little by installing a larger value cathode bias resistor in the power amp. Hopefully that will reduce (opr eliminate) the remaining thumping and I suspect that you will still have a good sounding amp.
Regards,
Tom

[slidincharlie (Carlo P)]

Possibly the best compromise so far...

I have tried four different NOS tube pairs: two old RCA that were in the amp, two old Delco-branded tuvbes (RCA and Sylvania), two ST-shaped Westinghouse and two JAN Philips.
The strongest tubes (Westinghouse and Philips, which were never used before) have also the strongest thumping noise.
Second worst come the two RCA's, which are severely mismatched.
Last, and best (thumping-wise at least) are the Delco, which are a little bit mismatched (5 mA).
I have put the two Delco in my GA18-T for the time being, and considering that the tremolo is strong enough at 4-5 on the dial, with the volume almost fully cranked the thumping is barely audible when I don't play. While playing there is no problem at all.
The Delco's are biased not too hot (60% and 68% of the 6V6GT max plate dissipation with their stock cathode resistor), so I don't think I'll lower them beyond this point.
Ah, and using shielded wire on the grids of V1 helped a little bit with the thump-thump.

Thank you Tom for your comments!

[Tom Phillips]

...the tremolo is strong enough at 4-5 on the dial, with the volume almost fully cranked the thumping is barely audible when I don't play. While playing there is no problem at all....

Sounds good Carlo. I think you are now getting about the best performance you can expect for that amp.

Cheers,
Tom