Marwatt build thread…

[Raoul Duke]

Got it, thanks Mike!
This is what I came up with:

Marwatt board rev5.1.png

The 5k1 is a conservative estimate on my part. Also changed the .68uf caps to a Philips MKT box form due to their more manageable dimensions (had some laying around) as well as the plate resistors to 1 watt per the schematic. Not sure it's necessary, but I figure Mr Swanson spec'd it that way for a reason. Added a bleeder to the reservoir cap also.

[cdemike]

Almost there. The cap needs to filter the new node, so it should connect to the other side of the 5K1 resistor. So I'd just reverse the jumpers' positions.

Something you could test while building is to have it without the added filtering stage to see if you encounter stability issues with the stock schematic. I'd be nervous with that many stages sharing a node, especially with the layout challenges that already come with placing a set of inputs in the center of the chassis. In the worst case you just add the node with the space allotted on the board. The rule of thumb I've read to prevent interaction between stages on either side of a B+ resistor is that the resistor should be at least 1K. Obviously higher affords greater degrees of isolation and stability.

[Raoul Duke]

Like this?

Marwatt board rev5.1.png

I might just try that test for the learning experience.

Thanks Mike!

[Raoul Duke]

Cabinet’s done. Guess I ought to start building the amp now…

Been researching and trying to “proof” my last layout before starting just because I’m a little unsure of what I came up with - so I decided to make the cab as a way of stalling. With its completion - I’m officially out of distractions/excuses to heat up the iron and start (other than the updated schematic that I work on here and there).

Last call I guess - any last minute fox paws in my last layout? Any corrections? Should I just shut up and do it, lol?

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Thanks in advance!

[Raoul Duke]

As I start the mechanical assembly and continue to study - I’ve begun to think I may include a “Trainwreck Type 2” master volume in this build. Here’s why:

- I have a 2-hole 6V6 Super Bass build I got from a local builder years ago that is nearly unusable without its master volume; and its Type 2 doesn’t seem to detract from the overall sound.

- A master volume seems to be a pretty integral part of the Hiwatt sound as well as a common mod to the 18-20w Marshall circuits.

- The chassis already has a hole for it, and it’s positioned pretty well in relation to the board parts that would change.

- I have the parts on hand and haven’t built the board yet - so now would be the time to make a change of this sort.

Any opinions or thoughts about this idea?

Thanks!

[Raoul Duke]

Here’s about a week’s worth of work here and there. Put the MV in today (check out those tiny .4w 2M2 resistors) I think I’m pretty close to being able to test the power amp section. Probably move on to the board later this week.

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[Raoul Duke]

Been slowly making progress as “regular life stuff” allows me to. Trouble is that stepping away from without leaving myself a note as to where I was and what I was thinking about next is sometimes counterproductive.

Put the board together, wired-up the controls mostly, started making some power connections intending to test that section before moving on and forgot to determine where my stopping point ought to be. In the below photo, I’m thinking I can lift the top end of the 2k2/3w dropping resistor and I should be able to test the AC coming in and heater supply, bias voltage and sweep, and B+ at the power tube sockets without voltage going forward. Looking for confirmation or suggestions to accomplish the same tests if I’m wrong.

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Any advice is greatly appreciated! Thanks!!!

[Raoul Duke]

Ok, actually finished all the wiring today and plan to throw it on the limiter sometime this weekend and see where I stand. Hopefully I have everything correct. This is the first build where I’ve done some re-design and layout work; and aside from the altered power section schematic I put together and posted on page 1 - my build follows the Huss schematic (also posted on page 1) to the best of my ability to read and follow schematics.

Didn’t realize that Mr Huss had created that schem until I found some old posts attributing it to him. I found the schematic on 18watt and I don’t remember him being credited there - but may have missed it. Either way - credit and thanks to Mr Swanson and Mr Huss for their work with this circuit. I’m hopeful that it sounds good - assuming I did my part right.

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More to follow; wish me luck.

[Raoul Duke]

Hooked up the limiter and variac and brought her up slowly stopping at halfway (~60 vac) then on to 120. No smoke, no noises, no smells. Limiter bulb (60w) filament barely glowed as it came up and pretty much disappeared after running for about 2 minutes.

Unloaded voltages at 120vac:

Heaters: 6.6vac
Bias: -15.3 to -31.4 vdc
B+: 381.6
B+1: 381.4
B+2: 381.2
B+3: 381
B+4: 380.4
B+5: 380.4

V1 plates: 377.3
V1 cath A: .001
V1 cath B: .001

V2 plate A: 376.5
V2 plate B: 376.6
Both cathodes: 0

V3 plate A: 378.4
V3 grid A: .012
V3 plate B: 378.3
V3 grid B: .006
V3 cathode: .003

V4 plate: 381.9
V5 plate: 382.2

Bleeder has voltage below 3vdc at 2 minutes after shutdown.

I think these look good enough to try tubes? My main concerns are that B+3,4,5 haven’t really dropped much and V4 and 5 seem higher than I expected. Do these look abnormally high?

Any opinions are welcome and appreciated!

[martin manning]

With no current draw the B+ string will be more or less constant. You’ve confirmed no shorts, so I’d set the bias to most negative, add tubes, and see if you can get it biased.

[Raoul Duke]

Thanks Martin! I’ll try that and post results as soon as I can.

Always exciting lighting a new build up and seeing it come to life!

[Raoul Duke]

Ok,
Tried 2 sets of tubes and got slightly different readings but a common result. Here’s the numbers:

Set 1: Mesa 6V6GTA STR-417
Plates: 368vdc
Bias at max negative side of pot: 28.4mv

Set 2: TAD 6V6GT STR
Plates: 365vdc
Bias at max negative side of pot: 30.2mv

I’m inclined to stay with the TADs unless there’s a reason not to?

Thanks!

[martin manning]

I’m guessing that 2 mA and 3V difference is just one pair of 6V6 vs. another. You could adjust the bias to match any desired idle current with either pair. I would choose by ear, if you have a preference.

[Raoul Duke]

Makes sense, thanks Martin. Once it makes noise - I’ll do just that.

Quick question:

Just noticed that with no tubes, my bias pot sweeps from -32 CW to -15 CCW measured at the OT/grid stoppers connection.
When I put tubes in; the pot sweeps 30mv CW to 50mv CCW measured at the bias sensing resistors.

Why would that be? I have a 2m2 safety resistor on the bias pot as well as the dual gang MV pot; would that cause it? Did I cross the plate wires up? I didn’t think so and kept all the color codes the same to try and prevent that (i.e. the brown and blue).

Puzzling (to me). Trying to search this now and not getting anywhere. Any clues are appreciated!

Thanks!

[martin manning]

Just noticed that with no tubes, my bias pot sweeps from -32 CW to -15 CCW measured at the OT/grid stoppers connection.

This is volts DC as produced by the bias circuit.

When I put tubes in; the pot sweeps 30mv CW to 50mv CCW measured at the bias sensing resistors.

This is mV DC at the cathodes, which, with 1R current sensing resistors is 30 mA to 50 mA cathode current, which is what the tube allows over the -32 to -15V range of grid voltage.