Couple of questions

[DG45]

Please go easy on the novice

I posted a similiar version of this on another forum, but no joy.

Couple of questions. Please bear with me.

I did a 45 watt Marshall build several months back. I'm wanting to experiment with some things now. It's a TD kit which is kind of a mix of several Marshalls. GZ34, EL34's, split cathode. High value filtering (50 Plate, 50 Screen, 50 PI, 47 V2, 16 V1). 440 volts on the EL34 plates. Preamp voltages are lowish (by design) compared to some other voltage charts that I've come across for various Marshall clone builds and in line with others. I'm finding that it seems the various Marshalls were all over the place in terms of voltages, both for preamp and the B+. My amp is a little stiff sounding, but I've been happy with it. But I think it's time to play with some reversible mods and see if I like the changes.

The dropping resistor between the output tube screens and the PI is 22K. I temporarily clipped in another 22K in parallel which brought the preamp voltages up. It didn't seem to affect the plate voltages of the EL34's (which makes sense) but I forgot to measure the screen voltages. Which brings me to my first question ... I don't quite understand the relationship between the plate and screen voltages of the output tubes. From what I'm seeing by searching the net, the screen voltages seem to run maybe just a few volts below the plates. I don't know what affect that resistor change made on the screen voltage, as again, I forgot to measure. But seeing how most Marshalls seem to use an 8.2K resistor there and the screen voltages still seem to be just under the plates, I'm not quite understanding why changing that resistor value doesn't seem to bump up the screen voltage. Or maybe it does and I'm just missing something. As I mentioned, I know it bumps up the voltages on the PI and preamp tubes. Can someone briefly explain how that works in as laymen like terms as possible.

Also, I notice that some Marshall voltage charts show the plates of V1 to run about the same voltages. Others show pin 1 to run below pin 6 by a good margin. That's how it is in my amp. I'm assuming that it has to do with the cathode resistors affecting the plate voltages. Which would mean that shared cathode Marshalls would show the V1 pin 1 and pin 6 voltages to be the same (or close) where split cathode Marshalls would have a lower V1 pin 1 voltage compared to pin 6. Is my thinking right on this?

What I'm thinking about doing is lowering filter cap values for the different stages so it's a bit spongier, not as stiff. Maybe 32 rectifier, 32 B+, 32 (or 16) PI, 16 V2, 16V1 (or sharing a 16 between V1 and V2). Also considering raising the voltage some on the PI and preamp by changing the 22K resistor to a lower value.

Here is a link to the layout.
https://d1sjrnpi226dnf.cloudfront.net/s ... 1391458164

Thanks in advance to anyone patient enough to read all this and add their input.

[Tazin]

I guess the first question would be what is it about the sound of the amp that you'd like to change?

The sceen voltage is taken after the choke but before the B+ dropping resistor(s) (i.e. 22K or two 8K2's), so changing this resistor doesn't affect the screen voltage.

A split cathode design for V1 will have differing voltages on pins #1 and #6 if the cathode resistors are of different values (i.e. 820 ohm for V1a and a 2K7 for V1b). If the setup is a split cathode using 820 ohm resistors for both cathodes (i.e. 820R/250uF and 820R/0.68uF) then the voltages on pins #1 and #6 should be close to one another. Obviously, a shared cathode setup would be the same too.

[danman]

Tazin covered all of the bases pretty well. The dropping resistor is located after the screen so changing it's value only affects the preamp voltages. You can play with the value there to change up the preamp voltages a bit. Higher voltage to the preamp will give you a bit more headroom, cleaner chord sounds and a slightly tighter feel I have noticed. Lower preamp voltages will cause it to distort a little sooner, give it a spongier feel and a browner sound. It's not a huge change but you can tell a small difference.

[DG45]

I see now how that resistor change doesn't affect the screen voltages.

To answer the question ... I like the amp so it's not that I'm looking for something that much different. It just feels a little stiff, best I can describe it. On the flip side, the low end is ample but pretty darn tight, even with the amp wide open. I'd like to see if I can get a little bit spongier feel. I've read that higher value filtering adds bass but also tightens the bass response. I was considering lowering filtering but I don't want to end up with a sloppy low end either. Sounds like increasing the voltage on the preamp tubes may just stiffen the amp up more, so that may not be the way to go. Of course, everything is reversible. Lowest voltage is on V1 pin 1 which is at 135vDC (pin 6 is at 180 V). Lower than anything I have seen in different voltage charts, but seems to work OK.

The amp has a 47uf filter on V2 and a separate 16uf for V1. 50uf everywhere else, including the GZ34 (except, it looks like 100uf if the silicone diode rectification is selected). I wonder what the effect would be if I dropped the one cap on V2 down to 16uf. What affect do you all think that dropping the filtering to 32uf for everything except a 16uf V2 and 16uf V1 would have? Do you think I risk the bass getting too sloppy?

The amp appears to have borrowed aspects of several different Marshalls, kind of a hybrid. But at any rate, looking at lots of layouts and schematics (which I cant read very well) seems like the Marshalls with higher filtering were SS rectified. And the split cathode Marshalls were SS rectified. Kind of hard for a relative novice to get his bearings from looking at the Marshalls, seeing how they varied so much. So, I appreciate all the help I can get.

Thanks

[Tazin]

In general, lowering the filtering does make the amp seem less stiff. The terms "spongier, softer, more round, and even more sag" are normally associated with lower filtered amps. In your particular amp design it's tough to say how much effect lowering the filtering will have since the amp has features such as: a higher number of B+ dropping resistors along with a 2K7/0.68uF split cathode V1 and 0.022uF output coupling caps. The 2K7 resistor and the 0.022uF caps are going to keep the bass in check therefore reducing the "flubbing" effect. Anyhow, when it comes to an amp with the higher number of B+ dropping resistors I personally like a 32uF-32uF preamp filtering which has been the norm in Marshall's since 1967. As for mains, screens, and PI filtering I like 32uF-32uF for both or 32uF-32uF for the mains and 50uF-50uF for the screens & PI.

[DG45]

Thanks Tazin.

I should consider giving it a try and see how it works out.

If I understand it, the orange wire from the 50uf section of the one 50+50 cap (lower cap in diagram) that is feeding the switch (tube/ss rectifier select switch) is paralleled with the 50uf from the other 50+50 cap (top in diagram) when the switch is flipped to activate the silicone diodes. So, the silicone diode rectifier gets 100uf. I think that's right anyway. But, I never figured out the 10K resistor between those two sections. Is that a dropping resistor?

Other than that, there is one extra dropper, as far as my limited grasp goes anyway. That one is between the two caps on the board, a 47uf and a 16uf, that feed V1 and V2.

Every time I figure something out on how this stuff works, there is something else that pops up that I don't understand. I don't have the best mind for this stuff. All I can do is keep at it. Thank you to folks like you and danman for the help when I ask for it.

[danman]

I have not seen the schematic but I would think that the 10k between filter sections is likely a balancing resistor. From your description it sounds as if the amp has 100uf main filtering with SS rectification and 50uf with tube rect. Tube rectifiers generally cannot tolerate as much filtering as SS rectifiers.

Are you happy with the feel and response of the amp when using the tube rectification or is it still to stiff? It may help if you could post all plate voltages for preamp and power tubes so we can see what you are working with. Also,keep a record of all voltages before you start making changes so that you will have a reference later on.

[DG45]

Hi danman.

I don't play it with the ss rectifier. I installed the switch in case the GZ34 goes out. I have tried it and it is stiffer with the ss diodes. The manual isn't clear but unless I'm seeing it wrong, I'm pretty sure you need to pull the tube to use the ss rect. I think it will work if you don't but you'd end up with both ss and tube rectification, like a Mesa, but you'd have 100uf on the GZ34. But again, I could be seeing it wrong. I'm not very savvy with this stuff.

There is a link to the layout in my original post.

I have the plate voltages jotted down already. This is with the stock 22K dropping resistor before the PI. The number in parenthesis is the voltage with another 22K clipped in parallel with the existing 22K. I didn't play the amp with it in. I just wanted to see what the voltages looked like with the lowered resistance.

V1
Pin 1 136 Vdc (165)
Pin 6 180 Vdc (216)

V2
Pin 1 145 Vdc (175)
Pin 6 258 Vdc (306)

V3
Pin 1 200 Vdc (236)
Pin 6 188 Vdc (223)

V4
Pin 3 445 Vdc

V5
Pin 3 445 Vdc

Thanks.

[DG45]

My thread here kind of died when the forum went down for maint.
At any rate, an update for anyone still following. I lowered the filter cap values from the 50uf's to 32's and from the 47uf at V2 to 16uf. I've read that going from 50uf down to 32uf may not make a huge difference, but it did in this case. Definitely "spongier", but too much so as the bottom end lost definition.

With the 32uF's in there I probably should have upped the pre-amp plate voltages to see what effect that had. Also thinking I should have tried a 30uf on V2 or gone back to the 47uF on V2 while I had the 32uF's in there. But, got impatient and reverted back.

When I get some time and motivation, I'll go back in and try those things. In the meantime, I'm enjoying playing through it.