Filmosound 621 conversion

[gilmo789]

Sadly, lockdown here killed my motivation to do quite a lot of things - finishing this amp conversion was one of them. I have basically got it sussed now and have wired ~ 75% of it. I was wondering what the learned people here could tell me about the input/first amplification stage as shown in the schematic. In the Guitar world article that I have been following, he basically gets rid of everything and wires the jack to the input (edit: by input I meant Pentode grid) with a 1M resistor across to ground. I understand this and will probably do the same.
I am a little confused by the pentode output. At the junction of the coupling cap and the volume pot there is what looks like a feedback loop going back into the input stage. If I wire the jack straight to the pentode grid then surely this is redundant. Is there any reason to keep it as is and use the loop on the input?

Here is my attempt to decipher the impossible to read circuit diagram. red are the values as given, green are mods, mostly as per guitar world article.

thanks,

[Enzo]

Redundant? The negative feedback inserts back into the grid CIRCUIT but not directly. But this is going to be a guitar amp, not a film projector, so the reason to keep or remove this local NFB is whether or not it sounds better one way or the other. So were it mine, I'd leave it there, as removing it is as simple as snipping one end free.

[loudthud]

R25 needs to be 100K, not 100 Ohms. See if you can find a clearer schematic. There are a couple of places where the blurry schematic makes it look like two wires are connected when they shouldn't be.

Try this: https://www.tdpri.com/attachments/be...21-png.803769/

[gilmo789]

R25 needs to be 100K, not 100 Ohms. See if you can find a clearer schematic. There are a couple of places where the blurry schematic makes it look like two wires are connected when they shouldn't be.

Try this: https://www.tdpri.com/attachments/be...21-png.803769/

I wish i had that 2 years ago when I started this, very useful. I think however, he has done what I have and recreated the circuit from the same bad copy I have. there are a few '?'s beside some of the values. I have installed the correct value for R25, I couldn't tell what the black blob was when I was making the markings

[gilmo789]

Redundant?

I mean in the sense that if simplify the input as per the GW article, then it is no-longer a loop. It just looks like it's sending some HF to ground (maybe a nice place for a switch?). There is quite a difference in the input resistance between the two options 47R vs 1.33M. Is this just a case of messing around to see what sounds good? Below is what I think the two variations are saying. I'm fairly certain that regardless, I can get rid of the 1nF and 20M because that is the input from the projector.

I've had to gut the amp and am rebuilding it as close to the original circuit

[Enzo]

I missed mention of removing the other resistors in your post #1 schematic. Suggest in future present schematic you intend. The article appeared to have none. Too much to wade through to draw my own. If I missed a link, please point it out.

Nitpick: 1.033 meg.

[gilmo789]

Duly noted.
I will experiment once I get it working. Thanks for the help

[gilmo789]

Going to update this thread with my progress in the hope that some day its of some use to someone.

I finished wiring everything and for now just went with the simple input circuit. To my great surprise it is basically working!

The Good so Far

• B+ voltages are all close to diagram values
• 21.7 VDC on the 6v6 Cathodes and 66mA around R35, the bias resistor. By my reckoning that gives PD of 11.5W which should do fine

And the Bad

• 100Hz hum very noticeable and increases with volume
• Pots pretty scratchy
• Tone circuit doing all kinds of weird things including surprising space noises but for the most part it shifts the treble and bass a bit
• voltage on cathode of PI (pin 3 of ECC35) in mid 50s as opposed to the 45V shown on diagram

Notes on the Hum
Filter caps seem fine (I swapped different caps in to check)
Pentode TC wire is shielded and shield is grounded (once)
Reading ~2VAC on the 4uF filter caps - Don't know if that is significant


I'm not sure it's as loud as it could be. It's difficult for me to tell since I've never owned a valve amp. I've had few big SS amps though (80-100W range) and they were pretty much intolerable in a room after about three on the volume knob.

Any thoughts, tips or comments appreciated

[Enzo]

4000 years ago when this thing was made, caps were expensive. That is why they use tiny 4uf, 8uf, 16uf caps. In a modern amp, I expect a few volts AC on 400-500vDC. first node. By the screen node, I expect a few millivolts of ripple. Farther downstream in t h preamp I expect ZERO ripple. If you have two whole volts of ripple there, yes that is significant. 4uf caps might be good, but that doesn't make them strong filters. My opinion is those caps should be larger.

You may have grounded the shield wire on the grid cap of the input tube, but your jack does not appear to be a shorting jack. Meaning the empty jack tip is not grounding, so it acts as an antenna. Plug a guitar in and turn the volume on it to zero, or just short the input jack. Does that reduce the hum?

Looks like your tone pot is going to have a volt or two of DC across it, is that the case? That will always result in scratchy pots. Even perfectly good pots. And check the volume pot, is there any DC on it?

[PURLABOY]

The 120pf cap you added in green is in the wrong place, it should be hooked up to the volume pot.