Note this is running with reduced voltages initially - I will order some new resistors to increase the voltages and that will give it some additional bite. I will also add the V2 mod and that will also provide bite (although at near unity gain).
So initial impressions:
So initial impressions:
- Can sound bluesy
- Can bark
- Chuffing loud (97dB/W and 2W)
- Does have some bass but it's not a bass amp.
I’ve managed to nail most of the hum, only two issues at max volume:
I will try the normal techniques:
- wide band hiss increasing the noise floor (not so much of an issue)
- a small amount of 50/100Hz hum, almost gone
- a spike at 20kHz that seems to be coming through the elevated heaters, so I think this may be some oscilation:
I will try the normal techniques:
- dc bench heater
- reverse the heater wires to give 180 from the B+ phase
- adjust the artificial tap resistances
- use a quieter B+ source for the reference.
Much better sound now - after a few mods and switching from attempting to use the 8R tape as the NFB to using the 16R tap for the speaker and the NFB.
Clear, clean (for a marshall) and probably over doing the NFB, but still sounding good:
I think now it's a case of ordering the parts to:
(b) two extra triode stages and attenuators for unit gain but shaping of the sound
(c) cold clipper changes
If the V2 mod turns out to be a loss, I may just double up the V1 and move the cold clip to the V2mod. That parallel doubling up should then reduce the noise floor of the highest sensitivity triode stage.
Clear, clean (for a marshall) and probably over doing the NFB, but still sounding good:
I think now it's a case of ordering the parts to:
- get the B+ voltages up
- to fix the 20kHz annoyance
- various mods for the sound
(b) two extra triode stages and attenuators for unit gain but shaping of the sound
(c) cold clipper changes
- improve shielding.
- FX loop
- 10% volume attenuator so I can crank up the amp to max and keep the neighbours happy.
If the V2 mod turns out to be a loss, I may just double up the V1 and move the cold clip to the V2mod. That parallel doubling up should then reduce the noise floor of the highest sensitivity triode stage.
Update.
So after a bit of experimentation and break in, I've been mulling over:
1. Going to increase voltages
There's a point when you want soft and a point you want crisp, it seems by playing quite a bit with the tube and solid state rectification I can get different sounds.
Solid state - using two UF4007 in series and also looking at using the 230V primary I can get a B+ voltage of 489Vdc. I can then drop down from there as needed. Allowing 390/350/335V B+ for pre-stages and still allow 275V plate for the EF80s. Testing this with the SS but not the primary change gives a good 445Vdc B+. I'll need to make some changes with the caps (the 22uF are 450V rated).
The sound really crisps up nicely but loses that bluesy/early rock tone.
As it stands the SS without the primary mod sits at around 263V EF80 plate, and when at full voltages should drive 1.75W (therefore we should be approx 100dB) - rock on.. Without feedback it's running +3-5dB.. it's uncomfortably loud!
Tube (EZ80) - with the UF4007 on the plate with 333R, has quite a low B+ voltage through the pre and power stages in comparison to the SS rectification. This gives it the more relaxed older bluesy sound through to the early rock sounds. This needs a little bit of tuning but I think I have a longer term solution.
Start up on the SS will have near 500V on the plates and the tubes Vo (cold limit) is 550V, so we still don't need a standby switch or soft start. I will make a switch to go between SS and tube rectification.
Next is the V2 mod. Thankfully I have enough PT power to maintain the higher voltages with two additional triodes. This will essentially make the pre stage:
V1 pre -> cold clipper -> V2mod a -> V2mod b -> V3 pre -> V3 CF -> tone -> LTP -> power stage.
Once that is in place I have a couple of options:
However I suspect that at least one of the V2mod triodes will get used as an optional switchable gain stage. The stage will simply switch the tube signal path into the chain or remove it and ground to reduce noise.
The switchable cold clipper is something I'll put in. I'm not sold on the Soldano distortion sound, preferring the fortin distortion sound which uses the tone stack with smaller coupling caps, a different sloping resistor, pots etc. However I don't like the fortin clean/mid distortion. So I may look at adding a switch for diverting the raw tone input into a different stack. I could do this externally with a bypass effects loop.
The diode clipping mod works, so that gives 1980s in buckets. So that will go on a switch.
I can't find anything more about bjt clipping but I will probably end up making a small bjt clipping board instead and experiment with that (see external fx loop).
I will put in a 16R attenuator that should be capable of 15W into the 1x12. I'm also thinking of a combo amp that is 24"W x 23"H x 11.5"D with off centre speaker with the baffle lent back a little. Looking a bit like an odd blues breaker.
So after a bit of experimentation and break in, I've been mulling over:
1. Going to increase voltages
There's a point when you want soft and a point you want crisp, it seems by playing quite a bit with the tube and solid state rectification I can get different sounds.
Solid state - using two UF4007 in series and also looking at using the 230V primary I can get a B+ voltage of 489Vdc. I can then drop down from there as needed. Allowing 390/350/335V B+ for pre-stages and still allow 275V plate for the EF80s. Testing this with the SS but not the primary change gives a good 445Vdc B+. I'll need to make some changes with the caps (the 22uF are 450V rated).
The sound really crisps up nicely but loses that bluesy/early rock tone.
As it stands the SS without the primary mod sits at around 263V EF80 plate, and when at full voltages should drive 1.75W (therefore we should be approx 100dB) - rock on.. Without feedback it's running +3-5dB.. it's uncomfortably loud!
Tube (EZ80) - with the UF4007 on the plate with 333R, has quite a low B+ voltage through the pre and power stages in comparison to the SS rectification. This gives it the more relaxed older bluesy sound through to the early rock sounds. This needs a little bit of tuning but I think I have a longer term solution.
Start up on the SS will have near 500V on the plates and the tubes Vo (cold limit) is 550V, so we still don't need a standby switch or soft start. I will make a switch to go between SS and tube rectification.
Next is the V2 mod. Thankfully I have enough PT power to maintain the higher voltages with two additional triodes. This will essentially make the pre stage:
V1 pre -> cold clipper -> V2mod a -> V2mod b -> V3 pre -> V3 CF -> tone -> LTP -> power stage.
Once that is in place I have a couple of options:
- It can add a defined gain/attentuated as needed
- It can add the shaping of distortion
However I suspect that at least one of the V2mod triodes will get used as an optional switchable gain stage. The stage will simply switch the tube signal path into the chain or remove it and ground to reduce noise.
The switchable cold clipper is something I'll put in. I'm not sold on the Soldano distortion sound, preferring the fortin distortion sound which uses the tone stack with smaller coupling caps, a different sloping resistor, pots etc. However I don't like the fortin clean/mid distortion. So I may look at adding a switch for diverting the raw tone input into a different stack. I could do this externally with a bypass effects loop.
The diode clipping mod works, so that gives 1980s in buckets. So that will go on a switch.
I can't find anything more about bjt clipping but I will probably end up making a small bjt clipping board instead and experiment with that (see external fx loop).
I will put in a 16R attenuator that should be capable of 15W into the 1x12. I'm also thinking of a combo amp that is 24"W x 23"H x 11.5"D with off centre speaker with the baffle lent back a little. Looking a bit like an odd blues breaker.