Kalamazoo Bass 30 project (first post)

[aab0mb]

Ok i measured 348 off the PT. What does that tell us about possibilities?

Do we need some pin voltages or is that enough to get an idea?

[bob p]

Interesting that a new user would call us "Ampagers" rather than "MEFers".
I haven't heard this place called "Ampage" in a long time. Too long.
I thought I was the only guy left who calls this place" Ampage" instead of" MEF".

[aab0mb]

MEFers seems slightly obscene to me. haha.

it's marked as ampage with a big A for the icon in my bookmarks. Old habits die hard.

Been a lurker here at times over the years but i'm getting my hands dirty a little more these days so i figured it was time to join the ranks and try to soak up a little more info.

Receiving a helpful and warm welcome so far. Nice place, this is.

[g1]

I thought I was the only guy left who calls this place" Ampage" instead of" MEF".

What??? You mean you're not using the "classic" forum?
AMPAGE - Forum
Kalamazoo Bass 30 project (first post) - AMPAGE Forum

[bob p]

Nope. The posting feature stopped working in what, May 2006?

[aab0mb]

Anyone have anything helpful to add now that i've got the first voltages?

[bob p]

i don't know what "348" off of the PT means. is that AC? 348-0-348? is that with a load or no load? naked wires, or hooked up to the PSU filer? tubes in or pulled? if the amp is functional, it would help to write voltages on the schematic.

[Jazz P Bass]

Does the amp, as it sits, work?

[bob p]

Getting back to the original question of what amp would be a decent one to build:

It would really help to know if the amp is working as it is, and what the voltages are in the working or non-working circuit.

Approaching the problem without this information, I finally got around to looking at the 7591 data sheet. I think your heater supplies have enough current delivery so that you can get by with either a 6L6 or a 6V6. The heaters probably won't be a problem.

In Class AB1 pentode operation with fixed bias, a 7591 can produce 30W with about 350V plate supply, and the numbers go on up to 45W with 450V plate supply. Cathode bias with a 200R resistor at 450V is rated to give 28W. Your amp looks like it's fixed bias with a little bit of contribution from a cathode balancing pot, so I'm GUESSING that your power supply might create about 350V in plate voltages, but I'm guessing. I'd really prefer to see some measured numbers.

Looking at voltages alone isn't going to give you the answer about which tube (6V6 or 6L6) would work best with this amp in a Fender-type conversion. You also need to figure out the primary Z of the transformer. The best way to get that number would be to take some measurements. The 7591 data sheet suggests that it might be in the range of 6600R. Are the speakers really two 8R drivers wired in series?

The amp has some interesting features. Like the tone circuits, which are a simplified baxandall tonestack and a HF bleed to ground off of the volume pot. Then there's a cathodyne PI that feeds balanced drivers for the output tubes.

This amp is pretty different in it's topology from the garden variety Fender circuits. I don't know what you really want to do with it, and there may be some temptation to gut the chassis because the 6EU7 are more commonly deployed in HiFi applications today than in guitar amps, because 12AX7, 6L6 and 6V6 are cheap and because 7591 are getting hard to find.

If this were my amp, my main consideration would be whether or not it works as it sits, and if it's not working, how much effort would be needed to get it working. When I come across interesting circuits like this, I like to indulge my curiosity, so I like to restore them rather than gutting them, to see what kind of sound they provide. The Fender sound is so ubiquitous that I'm not inclined to run off and gut everything that I encounter to build a Fender out of it. I'd like to see what kind of results you can get out of a circuit like this one. I'm not familiar with the sound of this amp, but who knows -- it might have an interesting character that makes it a keeper. It's worth a listen, anyway.

[aab0mb]

The amp makes noise but i think one of the power tubes is possibly bad. Doesn't light up at all. It distorts in a horrible way and the volume is low. I'm assume pulling the bad tube and probing the pins of the socket will give us some useful readings?

I appreciate the patience and support for a new learner. I'll do my best to pay it forward!

[aab0mb]

Pics of the "head"... And some gut shots once i got it out of that horrid cabinet.

The filter caps are kind of floating and held by some zip ties. Might have to do something about that... Everything looks pretty clean inside at first look. Whatever that's worth.

I think i have a cheap meter that isn't suited for this work and i need to get a "good" one. Some voltage readings:

7591 power tube #1 :

pins 1 and 8 not hooked up

pin 3 fluctuates and i can't get a stable reading on it which i think is my meter.

pin 4 reads 44.7

pin 5 reads .2

pin 6 reads -015

pin 7 reads 0


7591 power tube #2:

pin 1 is hooked up on this one and reads 46.7

pin 2 reads 0

pin 3 gives a fluctuation again and it's not stable. i saw an 85 sit for a second but.... uncertain.

pin 4 reads 44.7

pin 5 reads 56.7

pin 6 reads -015

pin 7 read 0

and pin 8 is not hooked up


Probed around the preamp tubes a bit and the plates have voltages on them from about 220 to 270 all measured. On the other side of one of the plate resistors i'm getting a stable 363 reading.

So is this indicative of my power transformer being something like 400-0-400? I'm new at this but i'm trying my darn hardest to get this thing to make some musical noise so go easy on me.

[aab0mb]

RCA 6EU7 Datasheet

Sylvania 7591 Datasheet


Data sheets for the pre and power tubes for reference.

[bob p]

http://www.mif.pg.gda.pl/homepages/f...127/7/7591.pdf

[bob p]

Those voltages don't say much, other than your plate voltages aren't reliable enough to measure.

Are you measuring AC or DC?

I'd trace voltages through the power supply and write them on the schematic. Doing that spares everyone from having to read the data sheet and saves us time.

[olddawg]

Swap the output tubes and verify if it is the tube or something else. Measure the voltages at the rectifier AC and DC.