The Much Anticipated (?) Bluesmaster Build Thread

[Raoul Duke]

All connected up as listed a couple of posts ago. Assuming I got it right - that’s the relays done. I preset the pot on the PAB to 100k to start with. Not much left before it’s time to hang some transformers on this thing. Made all but one co-ax cable (probably do it tomorrow). My previous 2 builds used Belden 1855A; but I’m using RG-174U on this build. Definitely easier to work with, I hope it sounds ok - not that I have much of a frame of reference, lol.

Fixed those two incorrect cap values as well today.

Feels like I’m getting there…

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[Raoul Duke]

Been a busy last few days, but the house is quiet today and I’ve been plugging away. Even though my time to work on this amp has been limited, I’ve used the past few days to go back and re-read a few BM build threads just to keep the mental wheels turning. I always discover (subtle) things previously missed that turn out to be very helpful - and of course the pictures are always worth many more than “a thousand words”.

First - went back and re-set the PAB pot to 68k. Seems to be a consensus that somewhere around this value is a good place to start fine tuning the PAB sound.

Next, hung the transformers and started looking at wire routing. I’ve found it’s worth taking a long look at this before hacking leads down and just connected them up. I’ve learned this the hard way a few times, lol…

Once satisfied I understood what needed to go where, began wiring up the PT and OT - which is where I am at present.

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Also, after looking through my notes - I’m thinking I may replace the 3k9 on the bias supply board. Both previous builds used a lower value and had 53v bias taps. This PT has 50v on the bias tap - so I’m thinking a lower value (maybe in line with my 102 build - which ended up 1k8) might get me closer to start with? Anyone’s thoughts about this are very much appreciated!

Moving right along.

[Raoul Duke]

Got this done last week and forgot to post it. Made the boards myself with Martin’s plans. One “new” thing I tried was using two larger eyelets for the two cathode ground points on the board. I think they are 3.5mm-ish.

Also, mounted the HRM using small nylon stand-offs and screws. Seems to work ok so far - I guess we’ll see when it’s time to get everything together and connected.

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[Raoul Duke]

Going over some details before doing anything else on the amp today and had a question:

Given that I’m building a 50w BM, should I be using the 8ohm tap for my GNFB? I did this on my 102 because of the change from 100w to 50w - so I’m thinking this principle would apply here as well.

Am I actually learning, lol?

Thanks!

[martin manning]

Yes, same deal. Double the secondary tap impedance or reduce the feedback resistor by 0.707.

[Raoul Duke]

Great, thanks Martin!

I went back and re-read that whole exchange between you and Jelle and I and figured it applied here - but just wanted to be certain because there was some confusion surrounding which resistor values I was using which I think muddied the water early in that conversation.

Thanks again! I appreciated the lesson six months ago and appreciate it now as well

[Raoul Duke]

Today I was able to finish up the transformers and wired the PS board. Might be getting close to smoke test time.
This time around I have an option for the first power-up: my trusty limiter or a recently acquired, economically priced variable transformer (which was more like buying a kit given the level of detail put into the assembly).

Back wall OT etc:

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PS board and surrounding area:

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Covered the cabinet yesterday and will post pics once it’s photogenic enough, lol.

If anyone sees anything I may have missed or done wrong - please straighten me out; I’d consider it a huge favor. Actually, any constructive criticism is always welcome and appreciated!

Thanks!

[martin manning]

A Variac and a bulb limiter is the best arrangement.

[Raoul Duke]

Plug the Variac into the limiter, amp into the Variac, then slowly bring the amp up Martin?

[martin manning]

I usually plug the limiter into the Variac, but I'm not sure that it matters.

[Raoul Duke]

Well, I’d rather do it your way - so that’s what I’ll do.

In other words - I trust your expertise more than my assumptions, lol

I’ll do some searching for a process, but any tips you have are very much appreciated Martin!

[martin manning]

I was trying to come up with a rationale, but in the end at 120V the current limit will be the same, Bulb Wattage/120

[Raoul Duke]

Thanks and understood Martin! I’m shooting for Sunday hopefully.

Got to take the bed off my truck and weld two new frame cross members in tomorrow. Anyone with a Chevy or GMC built in the last 20 years can probably relate.

Rather be working on the amp…

[Raoul Duke]

Ok; truck fixed - back to the build…

Smoke test results:

Plugged the limiter (40w bulb) into the variac and came up slowly.

AC flowing as expected from power in to fuse to switch, stand by switch works as it should.

Heater voltage was 2.9vac at pin 7 and 6.9 at pin 2 - but 6.2 measured across 2 &7; panel light consistent with this as well.

Relay supply out is 12vdc

B+1: 468.5
B+2: 468.5
B+3: 464.3
B+4: 449.4
B+5: 444.5

AC In at bias supply board is 345
AC bias tap in is 51.5
Bias out to bias pot is -69.7vdc

No smoke or smells, no hums; however - the limiter didn’t glow during operation. Just the expected “flashes” with the switches. Bleeders had everything under 1vdc within 3 minutes.

Any comments, opinions, or suggestions are greatly appreciated!
Thanks all!

[martin manning]

Heater voltage was 2.9vac at pin 7 and 6.9 at pin 2 - but 6.2 measured across 2 &7; panel light consistent with this as well.

Don't understand this... If pin 2 to pin 7 is 6.2, I would think pins 2 and 7 should both be ~3.1 to ground. Is the only load you have on the heater circuit the panel lamp? What kind of panel lamp is it?

AC In at bias supply board is 345
AC bias tap in is 51.5
Bias out to bias pot is -69.7vdc

All good. I would say AC in to HV rectifier is 345.

... the limiter didn’t glow during operation. Just the expected “flashes” with the switches. Bleeders had everything under 1vdc within 3 minutes.

Too little current draw to light it up.