Questions on old tube amp

[slidincharlie (Carlo P)]

Here I am again with my little Silvertone 1389 amp
It is working fine with its new isolation transformer. The tone is all there, although at a low volume (2-3 watts I suppose?).
I have replaced all the resistors because the original ones were out of specs. All caps were replaced by someone else before the purchase.
I attach here the schematic (mod. 1430 actually, but it fits perfectly my amp except for larger filter caps in my amp).

I have a few questions:

- I have negative voltage (-2.8Vdc) on the cathode of the power tube (pin 1), while the schematic (a different version) calls for 8Vdc. What can cause such negative voltage? What should I check? The cathode resistor and bypass cap are new. All other voltages in the amp are fine.

- I've been told in another thread that the part labelled "ARCO 462" is an adjustable capacitor. What is it for? How do I check if it's adjusted as it should be? What would happen if I adjust it differently?

- This schematic shows 40 and 20 uF filter caps, and 10 uF on the cathode of the power tube. Another version of the same schem calls for 100 and 50 uF filters and 30 uF as cathode bypass cap. What is the difference between using different values for these caps? My amp now has got 100, 50 and 30 uF caps. Is there any reason to switch to 40, 20 and 10 uF?

TIA,

[Enzo]

Look closely at the schematic. Note that what would have been ground for the power tubes is not actually chassis ground. There is a 68k resistor between that and chassis. if you are using chasis as the reference for your readings, that explains it. Measure boltage across the 150 ohm resistor. That will be the true reading. In fact use the bottom end of that resistor as the reference for all DC readings. The chassis is ground for the signal input.

Play music through the amp and FIND OUT what adjusting the ARCO does.

I cannot imagine any erason to take out the existing filter caps and replace them with smaller ones. If the amp works and sounds good, why change it?

[Rob Mercure]

Carlo,

Your volume should be low - these amps are usually around 1.5 W. Why don't you consult the TDSL on Duncan's website and familiarize yourself with the output tube specs? Oh, they do sound pretty good through a high efficiency 12" speaker.

As to your first question, I don't know - you shouldn't have a negative voltage there and I suspect that if the amp is working you're measuring it in a "different" manner <grin>.

Yes, it's an adjustable "trimmer" capacitor used to "null out" parasitic oscillation but feeding back a bit of signal out of phase - good old NFB - but not global. And from the small size of the cap affecting very high frequencies so you might not even notice any change if you mess with it - but you if you change it you might set up oscillation in your amp that can burn up your output section/speaker If you've got an oscilloscope then you would probably benefit from playing with it - probably "slamming" the amp with a high, fast signal and then looking at the trace for "ringing." But if you don't have a scope, and you think that the previous tech may have set it correctly, then "if it ain't broke, don't fix it."

For your last question, again why not go to the TDSL and see what input capacitance you can use for a 35W4 without damage. While the rest of the values aren't that critical the first one attached to the 35W4 cathode (40uf in your schemo) is.

Hope that helps - trying to use the old "teach a man to fish" (and I know that you know how to fish <grin> - caught anything strange and interesting lately?).

Rob

[gmoon]

Here I am again with my little Silvertone 1389 amp
- This schematic shows 40 and 20 uF filter caps, and 10 uF on the cathode of the power tube. Another version of the same schem calls for 100 and 50 uF filters and 30 uF as cathode bypass cap. What is the difference between using different values for these caps? My amp now has got 100, 50 and 30 uF caps. Is there any reason to switch to 40, 20 and 10 uF?

Carlo, I rebuilt a similar amp recently. 50L6 not 50C5, but the rest of the amp is much the same (and the 50C5 looks very similar, although not the output or max plate voltage.) Maybe it's wrong to compare the two, but I replaced the power tube cath bypass cap (originally 40uf) with a 10 uF, like the schematic (yours might be somewhat louder with a 30uF cap, vs the 10uF.)

In fact, almost all the changes I made to the amp (coupling caps, bypass caps, etc.) match, or are close to, those on the silvertone schematic.

The preamp's different, though. The cathode's grounded like the suppressor grid, and the plate resistor is 680K, not 330K. The screen and it's bypass cap, etc., is very similar.

No "ARCO" cap or nfb....

[slidincharlie (Carlo P)]

Thank you all for the replies.

- I did note that there was not a ground reference except in a few points (unlike in most other amps)... New readings as per Enzo's advice are ok now.

- A 50C5 is rated 5W max plate dissipation. The OT is tiny. This explains the low volume I suppose...

- I'll tweak a little bit with the adjustable cap to see if there is any audible difference with different settings (although I don't have a means to read capacitance). For now it is adjusted full clockwise. BTW, this cap is missing from a different version of the schematic that I have (attached here below).

- Re: filter caps.
The schematic in the starting post calls for 40 and 20 uF. The other version of the schem calls for 100 and 50 uF, which is what I have now in this amp. The 35W4 wants 40 uF as max filter input capacitor. Should I reduce the first filter to 40 uF and the second to 20 uF, or maybe keep them both to 40 uF?

- Rob, you have good memory but I'm not a fisherman... I'm into fish research... Yet on my next holiday in July I'll go in a small island with my speargun... I'll let you know if I catch anything good!

[Rob Mercure]

Carlo,

What I remembered was that you were a marine biologist or something similar. That's why I thought that you might have "caught" something different or interesting. I suspect that at some point part of your job is being a "fisherman" of some sorts but that wasn't what I was teasing about.

And with your background I'm sure that you didn't get out of grad school (I think you've got a grad degree?) without doing technical research that's why I was trying to help you learn to use the "tube manual" information (I learned to use the infomation in actual books). So, if you go back to the 50C5 in additon to the "plate dissipation" see if there isn't a specification for "power output" (if you can't easily identify the information look for specifications that are described using Watts as a unit - the highest one will usually be the plate dissipation and the second highest one the power output - the next one is usually the screen grid and sometimes - in older and/or industrial manuals sometimes there is one more which is heater which may be the highest one but which is traditionally described first in the listing). The "power output" will give you a close approximation especially for tubes such as the 50C5 which is almost always used in a manner identical to your amp. As to the 35W4 as long as you're not exceeding the maximum input capacitance you're in great shap - especially since the 35W4 only provide 1/2 wave rectification (and any power supply "hum" will be 60 Hz) - if you can post the schemo with 100uf first filter I'd be interested in seeing it as that should shorted the rectifier life.

Rob

[slidincharlie (Carlo P)]

Carlo,

What I remembered was that you were a marine biologist or something similar.

Correct!

And with your background I'm sure that you didn't get out of grad school (I think you've got a grad degree?) without doing technical research

Uncorrect! Italian universities generally give you specific background on the main topic (biology in my case), not in other matters like e.g. electronics, which is a pity.

that's why I was trying to help you learn to use the "tube manual" information (I learned to use the infomation in actual books).

Thanks, I appreciate your help and approach.

So, if you go back to the 50C5 in additon to the "plate dissipation" see if there isn't a specification for "power output"...

I got it... that's 1.9W.

As to the 35W4 as long as you're not exceeding the maximum input capacitance you're in great shap - especially since the 35W4 only provide 1/2 wave rectification (and any power supply "hum" will be 60 Hz) - if you can post the schemo with 100uf first filter I'd be interested in seeing it as that should shorted the rectifier life.

Rob, the schem is attached to my last post above. This one calls for 100uF at the first filter stage and 50 uF at the second. The other (hand-drawn) schem attached to the first post calls for 40 uF as first filter and 20 uF as second. Should I stick to the first one and respect the tube's ratings?

[Rob Mercure]

Carlo,

Sorry, I thought you had provided the hand drawn schemo again - and, by the way, whoever drew it did a damned good job! Hmmm, I dunno, one approach would be to see which amplifier schemo most closely approximates the circuit you've got and a second approach would be to use the "precautionary principle" and use the recommended limit. In this case since the schemo with the 100uf cap doesn't include the small trimmer cap I think the approaches are one in the same which leads to using the smaller input electrolytic - and the only reason to possibly use the 100uf cap is excessive power supply hum. Do you have such hum?

As to the other comments, while I wouldn't expect a marine biologist to have electronics training (although there are some fish, such as sharks and rays and catfish that navigate using electric fields) you did learn basic physics and, probably some statistics - no? And perhaps your physics background is different but in my physics classes I had to learn Ohms and Kirschoff's Laws which pretty much get's you started in electronics - and this was from a small Appalachian 4 year college (Clinch Valley College). So this sort of information is what I refer to as "technical research" (and I think you've got a graduate degree, don't you? For my Master's I had to do research on energy and water use and legal patterns and regression analysis and all sortsa funky stuff easily forgotten in the "real world" <grin>). So, again, I think you've got the brains and the background to forge ahead with learning tube electronics.

Rob

[slidincharlie (Carlo P)]

Carlo,
(...) Hmmm, I dunno, one approach would be to see which amplifier schemo most closely approximates the circuit you've got and a second approach would be to use the "precautionary principle" and use the recommended limit. In this case since the schemo with the 100uf cap doesn't include the small trimmer cap I think the approaches are one in the same which leads to using the smaller input electrolytic - and the only reason to possibly use the 100uf cap is excessive power supply hum. Do you have such hum?

Rob, there is some hum, considering also that this tiny amp has to be cranked to get some volume. Not excessive though. I will compare 40 vs. 100 uF to hear if there is any difference hum-wise.
The first schem ('hand-drawn') matches perfectly my amp (althouh it's mod.1430; mine is mod. 1389), except for the filter and bypass caps value. Yet these caps are new: maybe the tech who did the cap job had the 'printed' schematic instead of the 'hand-drawn' one? Or did he simply replace the original caps? We'll never know...
Considering that the 35W4 wants 40uF, I'll put a 40uF there and see if there is any hum issue. In case 100uF is better hum-wise, is it dangerous for the 35W4?
Thanks for all,

[slidincharlie (Carlo P)]

Towards an end of the story

It seems like I am at the end of this story...
- stepdown isolation transformer installed on the bottom of the cabinet. I put it below the rectifier tube, leaving some room for the power cord;
- first filter cap reduced to 40 uF, like per the hand-drawn schematic;
- there is still some hum that comes and goes randomly; anyway I have reduced it a bit using a shorting input jack: with a guitar plugged in the residual hum is very low. Maybe replacing the 15M resistor on the 12AU6 input grid with a fresh one could help?
Thank you Rob and the others!