Should I add a bypass cap in a cathode-biased amp?

[slidincharlie (Carlo P)]

The Silvertone 1457 amp-in-case is a small single-ended amp equipped with a 6X4, 2 x 12AX7 (one for the tremolo) and one 6V6.
This amp has got a 270 ohm cathode resistor on the 6V6 but NO bypass cap: the cathode resistor goes to ground with a 33 ohm resistor in series, and goes also to the cathode of the 12AX7 tremolo tube.
Should I add a bypass cap across the cathode resistor? Or does this circuit design not involve a bypass cap?
Also, this amp has got very low volume: compared to a Fender bf/sf Champ it is much less loud. The B+ is lower than the schematics (around 280V instead of 320V), and swapping the power tube did not yield any difference. The 6V6 is biased around 12-13W at idle.
The schematic is in attachment.
TIA,
Carlo

[Enzo]

The design is what it is. Chances are this tiny amp with tiny speaker sounded better without the increased bottom end the bypass cap would add. But one cap and a couple clip wires and you can find out instantly if you like it better with a cap.

You could bypass the cathode to ground, but i suspect you would prefer to bypass only the 270 ohm resistor, since the trem signal is inserted across the 33 ohm one.

I would check for DC voltage on pin 5 of the power tube, and also at the grids of the 12AX7 tubes. leaky coupling caps woulod not be surprising at all. And leaky caps puting DC on grids could be part of your low B+. Those and weak filter caps.

You swapped the power tube, but did you try a different 12AX7 too?

[slidincharlie (Carlo P)]

Thank you Enzo,
I'll try the cap+clips setup and read the voltages as per your advice.
I have already checked for leaky coupling caps - should I also check all other caps? (they are very few indeed).
I had already replaced the filter caps with fresh ones.

[Enzo]

I don;t know, Carlo, if the caps are already new, then they are not as likely to be a problem. certainly try more than one 6V6.

[slidincharlie (Carlo P)]

I don;t know, Carlo, if the caps are already new, then they are not as likely to be a problem. certainly try more than one 6V6.

Done. There is no more than a 10V increase in the 6V6 plate voltage with other tubes.
Thanks.

[gmoon]

Hi, Carlo

I have a Danelectro DM 10, and it's identical to the 1457 (except for the neon lamp.) It certainly doesn't lack volume or bass response...but I run it through a 2X12 cab, since the original cab and speaker are gone. Maybe there are other issues with your Silvertone.

This amp has a very full sound, with a warm distortion in the mid ranges and limited high-end (attributable to the preamp biasing?) It's a real sleeper.

[slidincharlie (Carlo P)]

Enzo:
I have re-checked the .003 coupling cap on pin 5 of the 6V6 and it IS leaky: the more the amp warms up the more it leaks and the lower goes the B+.
Actually tonight the B+ out of the rectifier is on specs (mains AC may vary +/- 10V or more here), but it goes down as long as the cap leaks. In about two-three minutes the cap leaks around 6V dc and the B+ decreases by about 10-15V dc. Unfortunately I don't have another cap of the same value now, but I can check with a .002 or .0047 just to see if the voltages stabilize.

To gmoon:
I do want to play this tiny amp with its cool original amp-in-case cab, but I will test it with a bigger cab too. Anyway I don't expect much volume and low end from a 4-5W single ended amp... For sure it plays less loud than a fender bf/sf Champ.

[slidincharlie (Carlo P)]

With a fresh coupling cap (a .0047 instead of a .003) on the grid of the 6V6 the voltages are stable.
With a different 6V6 the B+ and plate voltage are a little higher.
What puzzles me is the difference between the B+ from the rectifier (314V) and the plate voltage on the 6V6 (300V). Isn't 14V drop a bit too much? In fixed bias Fender amps the drop is generally 2-3V only. Such drop is present with different 6V6's I've tried.
The other volt readings all over the circuit are fine.

I tried with a 25/25 bypass cap across the cathode resistor on the 6V6: there is a notable increase of volume and (less notable) low end. But, as Enzo had supposed, the stock speaker dos not handle that extra power and bass well. I'll keep the stock no-bypass cap configuration.

[guitician]

That 14V drop is from the transformer primary. I'd guess the DCR of that transformer is higher than fenders. It looks like that schematic shows those voltages, but I cant read them, too small of a scan.

[slidincharlie (Carlo P)]

Hi guitician,
if you click on the image it will display huge.
Anyway according to the schematic the output tranny has a 275 ohm resistance and a 10V drop (from 330 to 320 Vdc). I hadn't realized that. So yes, probably we are in the ballpark. I've just re-read the voltages and now I have 324V and 311V on the two sides of the output tranny. Oddly the new coupling cap seems a little bit leaky, too: I can read about 200 mV on the downward leg in circuit.

[Enzo]

The transformer winding has resistance. With power off, measure yours. But for now lets go with the 275 ohms. You have 14v across it? Ohm's Law then says:
I = V/R = 14/275 = 50ma. How close or far from the actual tube current is that amount? That seems kinda high to me.

But looking at your schematic, I see 3.2 ohm secondary. That looks like impedance to me, not resistance. So if your primary has 275 ohms impedance, that is not the same thig as resistance. You can measure the resistance, and then teh currnets and voltages ought to fall into place.