New Bluesmaster, please comment

[soje]

Hi all.

I'm trying to get my latest (2. dumble) build to play ball (I guess making coffee is asking too much). The amp is based on the latest Bluesmaster schematic I could find in the files section. It's build in a gutted Randall RG50TC 50W amp, reusing the tubes, iron, chassis and combo box. The powertubes are Sovtek 5881 XWT.

Since the donor amp had 4 preamp tubes, I build it with a Dumbleator onboard, and since a lot of people speak highly of using a Minimix between the dumbleator and FX, I added this onboard also (the donor amp had a nice +/-15V supply on a separate board, ready for reusing )

After checking and fixing the initial problems like hum etc, I've had the amp running for halv a day to get it kind of settled. Now there's a few issues I need help to fix:


1a) The amp distorts way more than my #124 build on the clean channel (the volume pot was bad, so I just used to 470k resistors instead). Is this normal?

1b) Also the OD channel has a lot of distortion at hand. Working the knobs I can get some mild OD, but heavy shredding is easily at hand . Normal?

2) The amp is very dark and muddy with humbuckers, so I'll need to work on the voicing, but I'll try to fix the distortion problem(?) first.


Here are the voltages (by the way, it uses a gyrator, as the amp was born with no choke).

B+1: 476V, B+2: 435V, B+3: 404V, B+3(d-lator): 394V, B+4: 330V, B+5: 295V

V1
Pin6: 185V
Pin8: 1,61V
Pin1: 191V
Pin3: 1,56V

V2
Pin6: 207V
Pin8: 1,82V
Pin1: 213V
Pin3: 1,7V

V3
Pin6: 278V
Pin7: 18,4V
Pin8: 33,8V
Pin1: 248V
Pin3: 1,87V

V4
Pin6: 257V
Pin7: 25,3V
Pin3/8: 39,2V
Pin1: 249V
Pin2: 26,3V

V5
Pin1: 36mV
Pin3: 476V
Pin4: 433V
Pin5: -44V

V5
Pin1: 34,8mV
Pin3: 476V
Pin4: 434V
Pin5: -44V


Please comment
Sorry for the long post

/soje

[Structo]

Nothing jumps out at me, voltages look pretty good.

Make sure that you have the plates and cathodes wired correctly on V1 and V2.
For example, if you have a plate resistor of 150K on pin 6, make sure you have the 2K2 on pin 8.
I had my cathodes backwards on my amp and it sounded all distorted.

If it were me, I would have gotten the amp working properly first then added the Dumbleator because you may have a problem there and not realize it.

If everything checks out visually, take another amp that has an FX loop in it and plug the preamp from that amp into the return of the new amp to check the power amp.
And to check the preamp, reverse things and plug the send from the new amp into the return of the other amp.

[T Wilcox]

Have you swapped tubes around?
FWIW I once had a tube in my D-lator go out and the symptom was the clean channel getting all gainy and overdriven along with some drop in volume.

Todd

[Structo]

Yeah I should have said that as well.
Make sure they are known, working tubes.

And you can always bypass the D'lator with a jumper cable to see if the preamp and power amp are behaving.

[soje]

Thanks for the suggestions.

I already did a bit of tube swapping, but tried it again just to be shure, but nothing changed.

I did however have the filter cap for the CF sitting right on the tube socket (for testing) and grounded close by. I removed the cap, and it seemed to remove quite a bit of ghost notes (low level distortion, getting audible as the tone dies out).

I'm not entirely shure the gyrator is working as intended, so I'm gonna look into that.

I'll keep you posted, please comment if you have any further suggestions

Regards
soje

[Structo]

OK, so I gotta ask, what is a gyrator?

When I look it up it says it is an IC circuit made to simulate an inductor.

If you have such a thing installed on or in this amp then all bets are off.

Don't you think you should get the amp running first before adding mods?

If you are wanting to imitate or simulate a choke, try a 300 ohm 10 watt resistor where the choke would normally go.

You will have to check your voltages down stream from that.

[soje]

It is as you found, a simulated choke. Since I didn't have the resistor at hand, but does have the few components to make the gyrator, I went that way.

The voltages written are with the gyrator. However, there IS something wrong with the gyrator circuit I set up, so I'm working on that now. When it does work, I'll get a 300R resistor to test it against

Thanks for the input!

Regards
soje

[soje]

Uhm, by the way. In fear of asking stupid questions: Why does the resistor need to be rated for 10W?

How many amps are the preamp/PI/screens pulling?


Regards

soje

[soje]

So, I finally threw out the gyrator and went along with a resistor

Next problem:

The PAB is muddy. On my #124 build the PAB is much more in the mids. Here, the PAB is darker sounding (muddy) than both mid-boost and rock setting.

With PAB engaged, the mid pot does nothing, the bass changes slightly when turned all the way around, and the treble pot works the same as without PAB.

I've used the PAB cicuit shown in the schematic, with a 82k resistor from the mid pot to ground. In PAB mode the resistor is shorted by a vactrol VTL5C1. I tried changing the resistor to 220k. It changed the amount of boost, but the sound was still muddy.

Any ides what causes this?

Regards
soje

[odourboy]

I'm not quite sure which observation your question is referring to, but I the PAB circuits work quite differently in in a BM versus a Skyliner (assuming that's the stack you built your #124 with).

In PAB mode, the Skyliner is coupling the signal though the 390pF cap, which causes bass attenuation starting at about 1KHz. OTOH, the different topology of the BM stack PAB means there is very little bass attenuation with a BM stack. The overall result is a big midboost in the Skyliner stack with PAB versus a full boost with the BM stakc and PAB.

Playing with that resistor (82K, 220K... 22M) is basically like varying the amount of stack ground lift. A big value means lifting the stack more out of the picture and thus more boost (and perhaps more mud, if you think the bass if contributing to muddiness) and less effect of the stack control settings.

[soje]

Thanks for your answer, it answered my question in part.

The reason for the question, is that several threads point out, that the HRM OD channel, should be run with PAB on, so you don't have 2 tonestacks attenuating the signal.

However, as my Bluesmaster is now, PAB is not really helpfull in either clean or OD. It's too muddy. I've been over the connections a dozen times, but things seem to work as intended, except for too much bas/low mid.

Actually the OD channel is way better without the PAB engaged, since I can dial out the bass.

Maybe I should try lowering the cathode bypass cap of the first stage?


Regards
soje

[odourboy]

You are not the first person to report excessive bass issues with the Bluesmaster (although you might be the first to report the problem as being specifically PAB related). I've built several and never run in to this problem so I can't help you other than suggesting you once again review your wiring of the PAB circuit - maybe try to confirm the schematic you're using is right.

[soje]

It is indeed a bass-heavy amp. However, I really enjoy the fatness of the tone when playing the clean channel in jazz mode.

Which schematic is the latest by the way? I forget which thread I found it in.

I have a question relating to the mid/bass voicing of this amp. It seems the bass and mid pots are working in the almost in the same frequency era. However, with mid dimed there is very little change in turning the bass knob. Is this the way it's supposed to work?

I've played a little with cathode bypass caps, and coupling caps. I focued on the first 2 stages, since I wanted the PAB to sound good before going on to the OD. Would changing the coupling cap to the PI help controlling the muddyness of the preamp?

Thanks for your input!!!
soje

[Structo]

This schematic is interesting in that it shows how to link the PAB to the OD channel so when you footswitch the amp into OD, the PAB is on so that you don't have two tone stacks on.