Amp Build Report - Micro-Bassman tube amp in Marshall Lead 12 SS

[klooon]

I´d build a real Tweed Bassman, period.
No master , a real PI , meaning a long tailed one, not a cheesy cathodyne, and, most important, a couple power pentodes.
If you want low power, fine, forget 6L6, 6V6 and even EL84, but use a couple 6AQ5 or to simplify things, a couple ECL82, where you have 2 pentodes and 2 triodes (LTP wink wink) in just 2 small bottles.

Even if you forgo push pull out and use a single ended pentode stage, á la Champ/AX84/etc. you will still be closer to original Bassman sound than if you try to drive speakers with soggy noodle triodes.
And small signal triodes at that !!!!

Thanks! I really thought this could be a nice step up from the FireFly which I built in a tiny BB enclosure and that I really liked. Also an ECC99 and one 12AX7. Lacked 'only' a bit of bottom end, but else it was totally fine. I thought it had to do with the OT, which was on the border (current spec) for the ECC99 to bias well. Therefore this build. I also build a proper 6V6 amp [Supro amp build link], so don't worry about that ;-)

I will have a look at the ECL82's or 6AQ5's which I am not so familiar with... For now I try to stick to the current (tube) set-up, but maybe I have to do a sort of overhaul in terms of design. Maybe first try bit by bit to improve things and learn... maybe start by ditching the cathodyne?... Maybe start with the fixed bias modifications here...

[klooon]

I think the problem with the PR is the low (240V) B+ voltage on the cathodyne which causes the cathodyne to clip before the output tubes reach full power so it's ugly cathodyne clipping you hear not power tube clipping. The cathodyne is powered from the 240V preamp B+ node. I think the solution is to move its 56k plate resistor B+ connection over to the unused 320V B+ node.

In this amp (the Bassman micro), I see that the PI is fed directly after the choke and has a relative high B+. Do you think it's clipping also here too early??

[catalin gramada]

Hi. Think those global negative feedback network did a lot of mess- normaly should. But maybe sound good...imposible to predict.
All stages decoupled by one cap?...hmmm.
270k into phase splitter grid ? why? it oscilate so bad ?

Nice build. Congrats!

[mikepukmel]

Not too far off topic, I hope, but a Marshall Lead 12 popped up in a local sale,cheap, around 75.00. In the past Id have passed it by, but after checking out klooon's way cool project, sounds like a good thing to pick up cheap. Its got a chassis that would fit a small tube ckt, a good cabinet, with a 12" spkr, and decent front panel. Thanks klooon! Great build!

[mikepukmel]

I thought it would be nice to share my last amp build. A Bassman Micro, designed by Rob Robinette [link] with one ECC99, according to this schematic: [Bassman Micro Schematic]. *)

I used an 80's Marshall Lead 12 solid state amp with a 10" speaker as a donor amp.

This is my layout:
[ATTACH=CONFIG]42736[/ATTACH]
it includes some voltage measurements/calculations.

*) EDIT: I see that the schematic has been updated. The version I used is this one (V2.6 10/2015):
[ATTACH=CONFIG]42759[/ATTACH]

Hi klooon, would you mind sharing some of your parts sources? E.g. the transformers?
Thanks!

[Leo_Gnardo]

Not too far off topic, I hope, but a Marshall Lead 12 popped up in a local sale,cheap, around 75.00. In the past Id have passed it by, but after checking out klooon's way cool project, sounds like a good thing to pick up cheap. Its got a chassis that would fit a small tube ckt, a good cabinet, with a 12" spkr, and decent front panel. Thanks klooon! Great build!

If it's working, kind of a shame to wreck it. The Lead 12 is a surprisingly good sounding amp especiallly for solid state, it actually carries a recognizable Marshall tone. And - you needn't attach a speaker. It works well as an overdrive stage, just take the line out and it sounds wonderful. I put one of these into a (big name rock star's) Bradshaw rack many years ago. Dunno if he still uses it but it sure was a good tone choice. Of course you could make a fresh chassis and use the box, take the amp & rackmount it, whatever. My rec. is find a busted one if you're gonna gut it.

[mikepukmel]

If it's working, kind of a shame to wreck it. The Lead 12 is a surprisingly good sounding amp especiallly for solid state, it actually carries a recognizable Marshall tone. And - you needn't attach a speaker. It works well as an overdrive stage, just take the line out and it sounds wonderful. I put one of these into a (big name rock star's) Bradshaw rack many years ago. Dunno if he still uses it but it sure was a good tone choice. Of course you could make a fresh chassis and use the box, take the amp & rackmount it, whatever. My rec. is find a busted one if you're gonna gut it.

Didn't know it sounded good, thanks Leo. I find them on craigslist from time to time, in the 75 to 100 range, but almost never find a broken on listed for sale. Iwill check, tho, thanks.

[brodie65708]

Hi there, I was wondering what you planned on doing with the parts you don't use. I'm looking to build amps and pedals and someone pointed me your way.

[klooon]

Hi. Think those global negative feedback network did a lot of mess- normaly should. But maybe sound good...imposible to predict.
All stages decoupled by one cap?...hmmm.
270k into phase splitter grid ? why? it oscilate so bad ?

Nice build. Congrats!

Thanks for the comment! I disconnected the negative feedback circuit and the sound is not only much and much louder also the overdrive seems raw but not as unpleasant as it was before. I have to try it more but so far it seems to be an improvement!

[klooon]

Hi klooon, would you mind sharing some of your parts sources? E.g. the transformers?
Thanks!

Of course! Please have a look at the schematic. I made some notes there regarding the parts. The transformers:
Power Transformer (31 euro): Toroidal 50VA; Prim. 230 V; Sec: 0-250V – 120mA; 6.3V – 3A (so no 5V tap for a rectifier tube).
Output Transformer (25 euro): Hammond 125C; Audio: 8W; Max. DC: 60 mA; Pri. Inductance: 5,60 H

As I am in the Netherlands I purchased them both from tube-town.net in Germany.

For the filter caps I generally use F+T caps. Mallory 150 caps, 1W carbon film resistors and Alpha pots. Further, I like Cliff jacks as they are not grounded to the chassis, which is ideal for my grounding scheme.

Anything else you want to know?

[mikepukmel]

Of course! Please have a look at the schematic. I made some notes there regarding the parts. The transformers:
Power Transformer (31 euro): Toroidal 50VA; Prim. 230 V; Sec: 0-250V – 120mA; 6.3V – 3A (so no 5V tap for a rectifier tube).
Output Transformer (25 euro): Hammond 125C; Audio: 8W; Max. DC: 60 mA; Pri. Inductance: 5,60 H

As I am in the Netherlands I purchased them both from tube-town.net in Germany.

For the filter caps I generally use F+T caps. Mallory 150 caps, 1W carbon film resistors and Alpha pots. Further, I like Cliff jacks as they are not grounded to the chassis, which is ideal for my grounding scheme.

Anything else you want to know?

Thanks Klooon!

[klooon]

Didn't know it sounded good, thanks Leo. I find them on craigslist from time to time, in the 75 to 100 range, but almost never find a broken on listed for sale. Iwill check, tho, thanks.

So far the ones that I got were not in good shape, despite of the description of the sellers. Either very scratchy pots, a scratchy or totally torn speaker. And the one that was in a reasonable shape, sounded very thin and overdriven very fizzy... You could dial in a bit of a sweet spot, but still far from excellent... So I had no problems converting them...

[klooon]

Not too far off topic, I hope, but a Marshall Lead 12 popped up in a local sale,cheap, around 75.00. In the past Id have passed it by, but after checking out klooon's way cool project, sounds like a good thing to pick up cheap. Its got a chassis that would fit a small tube ckt, a good cabinet, with a 12" spkr, and decent front panel. Thanks klooon! Great build!

By the way, the Marshall Lead 12 has a 10" speaker. The box is solid.

[klooon]

I will try to experiment and try different things I found after some reading on web. My "action plan":
1) Zobel network - I will get a 5uF (>50V) and a 8.2Ohm/5W-10W resistor. (I followed this calculator: for an 8 ohm speaker, the proper DC resistance of the speaker is about Re = 6.56Ohm ==> Rz = 8.2Ohm and with a Le of 0.35mH --> C ~ 5 uF).
2) Conjunctive filter - 2.2nF (3kV) and a 20k to 30k 10W resistor (1.2 to 1.3* the primary OT impedance (~21.5kOhm)).
3) or only a 2.2nF (3kV) cap across the primary leads of the OT (just like the FireFly, see this link)
4) Play around with the Phase Inverter design. Get rid/change values of the grid stopper, maybe Paul C mod (fixed bias mod) or something like that.
5) Maybe implement a Vox cut control...
6)....

I already tried a few things. As explained, I disconnected the negative feedback circuit and the sound is not only much and much louder also the overdrive seems raw but not as unpleasant as it was before. I have to try it more but so far it seems to be an improvement!

The zobel network and the conjunctive did not get rid of the fizz much, it just threw a blanked over the amp... I installed the Vox cut control after experimenting in place of the master volume pot and changes the high frequencies just a tad different as the treble control pot.

I still have to look at the grid stopper and PI design (PaulC)...

But I start liking the feel and sound of the amp!

[klooon]