First amp build SW50R clone

[lamp]

Hello!

I am starting my first amp build and wanted some feedback about the schematic before I actually put everything together. My goal was to make a clone of a Morgan SW50R, thankfully I saw a post from tubeswell where he created a schematic for a SW22R and I used that as the base for this project.

Some of the changes from the original schematic that I felt fine about were

• Switching from 6V6 to 6L6GC STR (using different transformers so shouldn't be an issue?)
• 500 pF caps instead of 470 pF
• 22 uF caps instead of 25 uF

However there were a couple changes I made that I wanted to check to make sure there wouldn't be any issues

• 200k reverb mixing resistor instead of 180k (not sure of the impact this will have)
• 120 pF 500 V cap instead of 1000 V (on input tube plates, 1000 V seems extremely high?)
• Remove standby switch
• Add 220k 3W bleeder resistor

After reading Merlin's books and his site I decided to remove the standby switch, and was thinking of adding an NTC thermistor (CL-70 most likely) to the mains supply to help limit the current surge when the amp first powers on.

I just wanted to see what people's thoughts were and if it seemed like I missed anything or if any of the changes would cause issues. I tried to mark any changes I made in red on the schematic (except for the tubes/removed standby switch).

Thanks!

[shooter]

maybe read through this thread for 6L6 operating conditions
6SK7 as a driver for a 6L6 SE AMP (el34world.com)


my rule of thumb for driving guitar PA sections, I want my AC drive signal => my bias voltage

[PRR]

> 120 pF 500 V cap instead of 1000 V

I don't see what that connection is even for. Cap from plate 2 to plate 1 looks like trouble. If not oscillation, then gross treble-cut. Where is that from?

1,000V because 120pFd is a very small value and in the old days it was not worth making a lot of different ratings. These days there's more distribution and more price pressure so now 500V is a stock part.

I think that bias supply has been cobbled from two different plans. The trimmer is installed like bias came from high AC voltage but the AC is taken from a tap through a low value resistor. It may be possible to smoke the pot when you trim bias.

[lamp]

I think that bias supply has been cobbled from two different plans. The trimmer is installed like bias came from high AC voltage but the AC is taken from a tap through a low value resistor. It may be possible to smoke the pot when you trim bias.

For the specific transformer I am using (Hammond 290ex) the tap for the bias is 53 V. Regarding the schematic portion of it I have attached a "gut shot" of the amp I am trying to copy (source: tubeswell). Looking at the pic I can see the 470R into the diode (I put a blue oval around it) and then the cap/diode to the pot so it does seem that for this amp that was the intended design. In my research I heard it referred to as a bias dropping resistor (pg 216 of Designing Power Supplies for Tube Amplifiers, Merlin Blencowe). I also see a brief mention here on his page about biasing (valvewizard.co.uk).

> In some amplifiers the bias is taken from a low-voltage tapping point on the transformer winding. In such cases no series resistor (or a very small resistor) is required.

But that doesn't necessarily mean it needs to stay if others think it is not needed/should be changed.

I don't see what that connection is even for. Cap from plate 2 to plate 1 looks like trouble. If not oscillation, then gross treble-cut. Where is that from?

From what I've read, I think this would be a plate bypass capacitor or "dull cap" and it seems like the intended purpose is to roll off some of the high end, but I haven't seen it implemented quite like this so I could be wrong. The source is the bottom right most yellow cap in the attached picture (I put a lime green oval around it) but it is hard to see what wires connect to that part of the board.

1,000V because 120pFd is a very small value and in the old days it was not worth making a lot of different ratings. These days there's more distribution and more price pressure so now 500V is a stock part.

Okay great, so as long as I decide if I want to keep it then it is good to know that I can use the 500V cap. I assumed the voltage there should not be that high (since B+ should only be around 450 V in the first place) but wanted to make sure. I might end up putting it in and then doing some tests with and without it to see which I like better.

[PRR]

I was told wrong. In 1955 the usual rating was 500V working, 1000V test. I think the testing is more refined now. I do agree that 500V ought to be way ample.

(Note also that these are "new RETMA values" 27, 47, etc, instead of 25 and 50 etc. Was never sure when 'rational' value sequence happened.)

[lamp]

maybe read through this thread for 6L6 operating conditions
6SK7 as a driver for a 6L6 SE AMP (el34world.com)


my rule of thumb for driving guitar PA sections, I want my AC drive signal => my bias voltage

Just to make sure I look at the right things, it seems most of the discussion in that thread is about switching out the tube before the phase inverter, is that what you are suggesting as well? That a 12AX7 wouldn't be able to drive a 6L6GC?

[shooter]

you have 2 gain stages, but the 2nd one mostly recovers everything the TS sucks out.
the datasheets show from 20 to 45VAC max into the PA grids.
so without doing the math it appears you might be lacking.
It will still be loud, but getting much compression from the PA, might need a deeper look

[lamp]

you have 2 gain stages, but the 2nd one mostly recovers everything the TS sucks out.
the datasheets show from 20 to 45VAC max into the PA grids.

I'll try and work through some of the math or find a modelling tool to help me verify this. At first glance it seems somewhat similar to the normal channel on a Fender Super Reverb AB763 as I believe a 7025 and 12AX7 are the same tube. If so then that gives me hope that it will work but I will try and actually verify first.