Vetruvio class-A

[neazoi]

Hello all,
I bought a ready made amplifier from an Italian living in Lithuania. It is named Hermes 10 amplifier and it is a class-A 10 Watt hybrid amplifier, using tubes for RIAA and line and transistors for final amp.
I attatch you the schematic here.

I was wondering if I can replace the 1uF capacitors in the line stage with a 0.68uF ones (because the last ones I have are of much better quality) without degrading too much the sound.

Also I would like you to propose me any other modifications for the tube section as the bass region is not too loud, especially for the RIAA sercion which is a bit bad.

[Jeb-D.]

Assuming the schematic is accurate.
You can replace the capacitor between the the rotary switch and the 1M pot.

On the Phono input, you can replace the input cap with a jumper. I'd also change the 100k resistor on the phono input to match the recommended load of your phono cartridge, if different.

[kenpeter]

I know less than nothing bout RIAA,
but seems to me if you replace 1uF
coupling caps with 0.68uF, there may
be even less bass! Or perhaps the
low corner is set somewhere else,
and I've misunderstood this circuit???

I'd leave your 1uF in place, and add
the high quality 0.68 in parallel. This
should be an easy experiment with
short clipleads to see if it has the
desired effect before any soldering.

[kenpeter]

The output stage also looks a bit goofy. Like they
were intending an SRPP of Triodlingtons, but forgot
to couple the collector of the lower NPN back to the
plate of the lower triode?

This lower triode in voltage controlling position of
this SRPP is sadly unaware of actual ouput state.
And the rule of Mu can not inherit to the lower
Bipolar. If that was the intent, then its broken.

This could be fixed real easy with 1uF or so.
Bridging lower collector back to the lower plate.
Now both transistors act like high Gm triodes.

The triode in the upper position doesn't need
such coupling across the top, as both devices
in top half of the SRPP reference fixed power
rails. Critical coupling here for Mu inheritance
is across the emitter diode drop, +10 ohms.

[neazoi]

so you say just to connect the collector of the lower transistor to the anode of the lower tube with a capacitor? Have I understood well?

I do not know why he did why he did although it seems to work...

I will also probably parallel two 0.68uF caps to give more than 1.2uF so improve a bit the bass.

Does anyone have any idea of how to convert the amplifier to use a regulated psu without loading too much more the transformers?

[Wavebourn]

No need to parallel anode with collector; it should work well without it. The lower triode is just a cathode follower that drives a transistor that works as a current amplifier. Upper "trilingtone" is a follower with unity gain.
The thingy has a huge dynamic output resistance so your bass roll off is normal. Fatter caps can't improve your bass, they are already far fatter than needed.

However, paralleling anode with collector like Peter suggested you may introduce some negative feedback by voltage that may help with bass, lowering dynamic output resistance of the amp.

[neazoi]

how can I test these power transistors (2n3055 style) to see if they have burned out in my amplifier? the amplifier is the vetruvio presented here and it burns out the fuses at the collector of the transistors (fuse not shown in schematic)

Is it possible to have been overheat the transistors and short circuited them?

Anyway how could I check this?

[Wavebourn]

Yes, transistor may run away thermally since no compensation is provided. To check them just measure resistance between collectors and emitters; if it is zero they are dead.

[neazoi]

I removed them and measured them one by one, two of the transistors (the ones that connected with the +Ve) are dead indeed.
I replaced all of the transistors with 2n3055 but the amplifier does not work. It still again it has lots of hum (unregulated PSU) but I cannot hear any music comming out of it. I retuned one of the bias pontentiometers (by hand, without using mA-meter) and I burned out one of the fuses again.
Any suggestions please?