Bugs in a scratch built JTM45

[mort]

Uh oh. Might have another issue.

When I got home today, I plugged in (w/kt66's in) and had the channels bridged. I played at fairly high volume for about 5 minutes and POOF, the fuse went. I replaced the fuse , took the channel jumper off, and cranked the hell out of each channel for 30 minutes and no fuse blow.

Shouldn't I be able to bridge the channels on this amp? I'm not very familiar with Marshalls, Fenders are my usual thing.

Gonna get a whole set of voltage data here in a sec...


Here's a little demo I recorded just a few minutes ago.

https://soundcloud.com/mortatort/jtm45-amp-demo

[MWJB]

You can play with the channels bridged, but here's the thing...you changed the power tubes & in doing so, probably rebiased the amp? You appear to have got the bias down to useable levels with EL34 & 6L6...then subbed in KT66.

Traditional KT66 draw more plate & heater current than 6L6, less heater but more plate current than EL34. There is the Tung Sol "drop in" KT66 that has 6L6 style currents, but you don't mention brand.

You haven't as yet given us a bias idle current setting? Not a negative voltage, but a reading in mA/mv of plate/cathode current. You have a bias pot, but you don't mention any method for reading the current (like matched 1ohm resistors from the power tube cathodes to ground). My guess is that the bias is borderline with KT66, a big AC signal is enough to push the average current over the top.

[big_teee]

Uh oh. Might have another issue.

When I got home today, I plugged in (w/kt66's in) and had the channels bridged. I played at fairly high volume for about 5 minutes and POOF, the fuse went. I replaced the fuse , took the channel jumper off, and cranked the hell out of each channel for 30 minutes and no fuse blow.

Shouldn't I be able to bridge the channels on this amp? I'm not very familiar with Marshalls, Fenders are my usual thing.

Gonna get a whole set of voltage data here in a sec...


Here's a little demo I recorded just a few minutes ago.

https://soundcloud.com/mortatort/jtm45-amp-demo

Which fuse, you should have a AC fuse and a B+ Fuse.
If you don't have them, I would Install the 1 Ohm resistors on the power tubes so you can read idle current.
Are the KT66 tubes the ones that came with the weber amp?
Try Raising the negative voltage on pin 5 of the power tubes, and see that helps.
You are probably pulling more that the 1/2 amp plate current?
Also I would take the time and record a full set of Pin Voltage readings for all tubes.
It is good to have a record for future ref. if you have problems, you have something to compare to.
T

[mort]

All voltages vdc unless specified

122.3vac mains input
6.34vac to all heaters

V1
1- 264.0
2- .001
3. 2.06
6. 258.0
7. .002
8. 2.06

V2
1. 219.3
2. .001
3. 1.37
6. 366.6
7. 219.3
8. 220.1

V3
1. 275.2
2. 27.17
3. 46.4
6. 280.8
7. 29.0
8. 46.4

V4
1. .000
3. 505
4. 504
5. -67.5
6. 504

V5
1. .001
3. 505
4. 504
5. -67.5
6. 504

V6
2-8 = 5.39vac
4-6 = 759vac

2. 86.6vac
4. 378.5vac
6. 378.7vac
8. 83.1vac


KT66's are new Valve Art brand, and the EL34's are new Winged C. I had experimented a little with each and they both sounded ok in the amp without any adjustments. I'm guessing though, from the comments, that this isn't the right way to go about it?

I'm not sure how to determine bias current... link?

Oh, and bridging the channels had blown the mains fuse. The B+ fuse has not had a problem.

[big_teee]

So judging by the very high voltage.
You must not be using a Rectifier tube.
I guess your using the Copper top, or plug in Diode rectifier?
T

[mort]

No, its tube. A New shuguang 5ar4/gz34

[big_teee]

No, its tube. A New shuguang 5ar4/gz34

378AC is really hot.
My 2204, is 345AC
It produces 492V with diodes.
You may want to try a different rectifier tube.
See what the others say.

T

[mort]

Thanks big teee. I think I have an old 5u4g?? at home. I'll try it later when I get back.

[g1]

You might want to wire up the PT for 125V operation rather than 120V (blue instead of brown). This may tame the voltages a bit.

[mort]

You might want to wire up the PT for 125V operation rather than 120V (blue instead of brown). This may tame the voltages a bit.

This transformer doesn't have the 125v input tap. Just the Brown and black. I had ordered the weber PT, but they sent a Heyboer with the same part number. When I ordered, the weber showed to be in stock, but they must have been out because now they show out of stock.

[mort]

Maybe the PT is my problem?

I was looking back over the PT diagram (for the Weber W022798, mine is HY022798)

The Heyboer does not have the 125v input lead, for one. For two, it also does not have the red/white 660v taps, only the red 720v taps.

Can I remedy the high voltage situation without replacing the PT?

[big_teee]

That is a really hot Transformer without the 125v tap and no 660v Secondary tap.
Another thing on your layout.
Not sure what ohm speaker cab your using.
You have 4, 8, and 16 ohms for EL34s.
You have 8, 16, & 32 ohms for KT66s.
So what ohms is your speaker cab?
Also you mentioned you had a different Rectifier tube.
A 5U4 Maybe?

[mort]

That is a really hot Transformer without the 125v tap and no 660v Secondary tap.
Another thing on your layout.
Not sure what ohm speaker cab your using.
You have 4, 8, and 16 ohms for EL34s.
You have 8, 16, & 32 ohms for KT66s.
So what speaker ohms is your cab?

Oh yeah, I forgot all about the different ohm ratings when using KT66's. The cab is 16 ohms and I have the selector set at max ohms, which would be 32 for 66's.

[mort]

What are the odds that the missing taps are tucked under the end bell?

The Heyboer version of this PT is listed on Weber's site as having a 125v input tap... it doesn't mention the 660v taps on the secondary... and I can't find a schematic for the Heyboer. But with the same part number(basically) I would think they should be identical.

[big_teee]

Oh yeah, I forgot all about the different ohm ratings when using KT66's. The cab is 16 ohms and I have the selector set at max ohms, which would be 32 for 66's.

So you would want to bump that down a notch.
If you have it, try the 5u4 rectifier.
You probably want to start over with it and pull the tubes and read the rectifier voltage.
Do that before you put the tubes in it.
The limiter, also if you think you may blow a fuse.
T