Identify circuit

[shocki]

Can anybody identify the circuit this amp is based on? I see some familiar stuff. But I would interested what other people (who are more experienced than I am) think about that.
http://ivanrichards.files.wordpress....-rca35-005.jpg

This is a description of the amp and you can see the panel.
http://www.morganamps.com/amps/rca35/rca35a.php

Thanks alot.

[soundguruman]

It's just a basic copy of Fender or Marshall...Fender Bassman, etc...there is nothing different.

[shocki]

I am not sure if this is fixed biased ? Cannot see a trim pot or anything for adjusting the bias.

[Justin Thomas]

Agree w. SGM - single channel BF Bassman/Fender Normal channel. Cathode bias though, judging by big honking resistor & lytic tied to pin 8/1 of the power tubes and what appears to be a red wire going to the other one... Definitely something I would like, though I'd movechat cathode resistor away from the caps a bit more.

Justin

[soundguruman]

No, there is no bias voltage or adjustment.

[Tom Phillips]

I am not sure if this is fixed biased ? Cannot see a trim pot or anything for adjusting the bias.

The amp is cathode biased. The cathode resistor is that long black cylinder to the right of the filter caps in the photo you linked. The cathode bypass cap is right underneath it. The close proximity (actually in contact) of the cathode resistor to the electrolytic caps is bad construction practice.

[shocki]

Ok. Thank you. I am wondering if he does it for simplicity (and be able to use different tubes) or for the sound. The MV seems to be some kind PPMIV ?
So the power section is also "normal" blackface but cathode biased? Is this kind of power section also used by other amp builders? As far as I know cathode biased should sound a bit more compressed and should have less clean power.

[Tom Phillips]

...Is this kind of power section also used by other amp builders?

Cathode biased power sections are not uncommon for low and medium power amps. This applies to both old and new designs. Cathode biasing is also one of the design features that make it possible to use different tubes in the same amp without re-setting the bias. Several builders do this. However, I believe that you must accept a compromise in performance when switching tubes. The differences in sound seem to be OK to the players though.

[Enzo]

Every Fender clonish amp you see is used by someone else. These are just guitar amps, not high precision NASA lab gear. This guy has put together his favorite circuits, nothing unique. Fender made both cathode biased and fixed bias amps. Many amp makers use cathode bias, it is not rare.

[shocki]

Cathode biased power sections are not uncommon for low and medium power amps. This applies to both old and new designs. Cathode biasing is also one of the design features that make it possible to use different tubes in the same amp without re-setting the bias. Several builders do this. However, I believe that you must accept a compromise in performance when switching tubes. The differences in sound seem to be OK to the players though.

Yes of course I know its widley used. In some Vox, early Fenders. Maybe I asked this question wrongly. I am just wondering if this would be a good choice in a typical blackface kind of amp.

[J M Fahey]

In fact with pencil, paper and a mug of coffee plus no wifey/kids/pets/sports on TV you can draw its circuit in less than an hour.
Even though some wires disappear under the board, it's easy to guess where they go.
As in: pins 3 and 8 on the first dual triode on the right *must* go to nets 1K5//1uF and 820r//1uF on the extreme right, those are classic 12AX7 cathode biasing values, and so on.

[shocki]

n fact with pencil, paper and a mug of coffee plus no wifey/kids/pets/sports on TV you can draw its circuit in less than an hour.
Even though some wires disappear under the board, it's easy to guess where they go.
As in: pins 3 and 8 on the first dual triode on the right *must* go to nets 1K5//1uF and 820r//1uF on the extreme right, those are classic 12AX7 cathode biasing values, and so on.

True. I think I will try that.

[Tom Phillips]

...Yes of course I know its widley used. In some Vox, early Fenders. Maybe I asked this question wrongly. I am just wondering if this would be a good choice in a typical blackface kind of amp.

When you ask the question like that the answer is NO. It is not a "typical blackface." However, you seem to be in possession of the amp so isn't it better for you to play through it and decide if you like it?

[shocki]

When you ask the question like that the answer is NO. It is not a "typical blackface." However, you seem to be in possession of the amp so isn't it better for you to play through it and decide if you like it?

No I am not in the possion of it. I am thinking of building a small blackface amp and this looked nice. Maybe I am using cathode and fixed bias switch and see what I like better.

[Tom Phillips]

No I am not in the possion of it. I am thinking of building a small blackface amp and this looked nice. Maybe I am using cathode and fixed bias switch and see what I like better.

OK. Now I understand that you guys were talking about drawing the schematic from the photo. If you are going to build your own amp you can start with a know circuit of the amp you like best. Then tweak to taste including the cathode vs. fixed bias. Lot's of fun. There is probably nothing all that special about the circuit in the Morgan amp circuit that you referenced in the first post. It does look like a nice compact single channel amp concept. You can adapt many of the classic Fender BF circuits using the same approach.