Issue With Bassman 5F6A Build

[Guitarnut]

Vibratoking is right, you'll get there, and its pretty close.

The only thing that looks to be way out right now is the 1-2-3 triode in V2. You got ~820 ohms to ground, but do you see 1.78V or 0V at the cathode? The plate voltage is high so it is not at the right operating point. The other side (the 6-7-8 triode) should come into line once this is resolved.

The V1 triodes won't match exactly, and you have 10% tolerance resistors in there too.

Okay, I'll give a look when I get in this evening. Good to know I'm making prgress.

Peace,
Mark

[martin manning]

The pictures of the power tube sockets a couple of pages back show the plate and screen connections are correct. The screen resistors looks like 47k, but that could be the color balance making brown look orange.

[Guitarnut]

I've verified the screen resistors are 470￾Ω￾.

Here are some pics that I didn't post in the earlier series. The black Sharpie marks on the sockets are a pin 1 locator.

V4

[img:800:600]http://www.crenshawweb.com/bassman/build68.jpg[/img]

V5

[img:800:600]http://www.crenshawweb.com/bassman/build67.jpg[/img]

Rectifier

[img:800:600]http://www.crenshawweb.com/bassman/build69.jpg[/img]

Peace,
Mark

[Guitarnut]

Vibratoking is right, you'll get there, and its pretty close.

The only thing that looks to be way out right now is the 1-2-3 triode in V2. You got ~820 ohms to ground, but do you see 1.78V or 0V at the cathode? The plate voltage is high so it is not at the right operating point. The other side (the 6-7-8 triode) should come into line once this is resolved.

The V1 triodes won't match exactly, and you have 10% tolerance resistors in there too.

Thanks again Martin. I really appreciate you hanging in there with me.

Okay, we have 805 ohms at V2 pin 3 and the pin 3 side of the 820R. 0 ohms on the ground side of the 820R. R across the 2 leads of the 820R is 819. Voltage on pin 3 is 0v.

On your note about V3 grid voltage, the junction of 1M,1M, 470,10k, the voltage is 45.5. Resistance there 14.2K powered down.

Peace,
Mark

[Guitarnut]

Target practice might preclude using the parts again.

It 'looks' much better than before. It looks really nice. It looks like you took your time. If it doesn't work we will all be kind of pissed off at it with you.

Nah, you can beat this thing with Martin's help. Seriously, just keep working at it. It's a finite problem and you'll eventually solve the puzzle and have fun playing through a great amp that YOU built.

Thanks guys. Your support is much appreciated!

I'm a lousy shot anyway.

Peace,
Mark

[Reeltarded]

You get better after a few amps.

[martin manning]

Okay, we have 805 ohms at V2 pin 3 and the pin 3 side of the 820R. 0 ohms on the ground side of the 820R. R across the 2 leads of the 820R is 819. Voltage on pin 3 is 0v.

Something has to be open here. Can you get the voltage on V2 pins 1, 2, 3, and 4, 5, and 6 again? If pin 3 still shows zero, try another 12AX7.

[Guitarnut]

I changed out the 12ax7 in V2 but there was no change.

I took a break from that issue and wired the inputs. The Bright mix resistors are grounded through 1M resistors to the ground lugs of their input jacks pending the arrival of the 4th 12A jack. The Normal inputs are wired as Martin suggested in the modern Bassman wiring link with the first gain stage grounded there as well.

While I was in the area, I decided to move the V2 ground from the bus up to the jack lug where I grounded the Bright inputs and viola...all the numbers changed.

I think I may be ready to plug in a guitar!

V1
1 117vdc
2
3 2.99vdc
6 176vdc
7
8 3.01vcd

V2
1 185vdc
2
3 1.2vdc
6 328vdc
7 185vdc
8 185vdc

V3
1 272vdc
2
3 47.9vdc
6 243vdc
7
8 48vdc

V4
1
3 436vdc
4 436vdc
5 -50vdc
6 438vdc
8

V5
1
3 432vdc
4 434vdc
5 -50vdc
6 434vdc
8

[Guitarnut]

I plugged in my lovely Strat and turned up the Normal vol but I get zilch...just hum. Son of a...!

More chasing to do. Sounds like 60hz to me.

http://www.crenshawweb.com/bassman/hum.mp3

Peace,
Mark

[Reeltarded]

Dood.. It sounds like a cable with no guitar connected.

That's good!!!

With the amp on and the volume set medium low take just one probe from your meter and start at the input. Touch every joint along the signal path until you find the first place you get a real signal.

The problem is the thing before the first real signal is found.

[Guitarnut]

Dood.. It sounds like a cable with no guitar connected.

That's good!!!

With the amp on and the volume set medium low take just one probe from your meter and start at the input. Touch every joint along the signal path until you find the first place you get a real signal.

The problem is the thing before the first real signal is found.

I'll give it a shot!

Peace,
Mark

[Guitarnut]

Before reading the above post by Reeltarded, I checked the inputs by following Martins instructions from an earlier post and they check out fine.

Plug in Norm 1 (with 1M)
Hot to ground 999K.
Pin 2 to ground 1031K.
Pin 7 to ground 0.

Plug in Norm 2
Hot to ground 141K
Pin 2 to ground 74K
Pin 7 to ground 0.

Peace,
Mark

[Guitarnut]

Dood.. It sounds like a cable with no guitar connected.

That's good!!!

With the amp on and the volume set medium low take just one probe from your meter and start at the input. Touch every joint along the signal path until you find the first place you get a real signal.

The problem is the thing before the first real signal is found.

I should probably ask. Is the other probe grounded? What's the meter set for? Or, am I listening for something?

Peace,
Mark

[Reeltarded]

You are simply using the probe as a source for making safe contact and a tic generator.

Meter off. Umm.. Unplug the probe even. Watch the other end that it doesn't end up anywhere harmful.

[Guitarnut]

You are simply using the probe as a source for making safe contact and a tic generator.

Meter off. Umm.. Unplug the probe even. Watch the other end that it doesn't end up anywhere harmful.

Got it.