Newbie looking for help on 5E3 hybrid

[Jetav8r]

This is a modified 5E3 Kit amp I recently purchased from a local designer. It has never worked; it powered up but never produced any sound. The designer helped me put it together as part of the deal. He wants more money to trouble shoot what ever it is that might be wrong. I thought I would throw it to the experts and see if anyone on this forum could help me. His schematics are hand written and difficult to read but if need be I can scan and attach. I was hoping this layout picture would be enough to get me started on trouble shooting. I re-soldered the connections on the board and other areas that looked shoddy per his recommendation. After doing so I fired it up and approximately 1-2 minutes later the annotated cap blew. Other specifics, Weber WPTGP power transformer, Weber OT, (2)6V6 GT power tubes (2) 12AX7 preamp tubes, 500k volume and tone pots, solid state rectifier, Weber signature series AlNico 12S speaker. Any suggestions would be greatly appreciated, i.e. paper weight, conversation piece, scrap metal?

[überfuzz]

Did it ever work as a modified amp? If I where you I would stick to the original 5E3 circuit. Later on when you're the expert you can start modify the amp.

[Jetav8r]

He claims it has. This is a new learning experience/hobby for me. This circuit is the designers idea not mine, claimed to be similar to a 5E3. Thanks, I agree with you, stick with a proven design.

[JHow]

Was the cap that blew reversed polarity?

Have you eliminated the usual suspects: speaker, speaker wiring, tubes all light up?

Have you measured any voltages? You say it powers up - by that do you mean the pilot light comes on?

[Jetav8r]

JHow, the polarity was in order with the rest of the caps, nothing reversed, my speaker is brand new but I haven't been able to test it on anything else because I have nothing else. When tested on the sellers speaker the result was the same, no sound. The pilot light worked, tubes lit up, as far as the voltages go we checked without tubes in and they were good. Even though I have a burning desire to learn, due to my inexperience I think I'll have to leave this in the hands of a professional until I gain more knowledge. Thanks for your help.

[JHow]

Probably the best course of action (take it tech), although if your tech is also the builder, then I might haggle with him a little bit.

I don't see anything obviously wrong in the picture, but I can't see all the leads. You may have some hum issues one you do have this one running.

[ThermionicScott]

Care to post up scans of the builder's schematics?

[Bruce / Mission Amps]

It looks like a single channel version of a 5E3... with some power tube mods too... a schematic would help though.
Man that's odd wire colors going to the SS FWB rectifier and out to the main B+ filters.
What power tranny is this with those red and blue/red colored hi-v secondaries... blue/red or blue is usually a lower voltage secondary, like a bias tap... but who knows.
I'll look at it again later....

[JHow]

Looks like blue w/red is a 410V tap and red is 0V, or else 690V. OP doesn't say what voltages he has. Lots of options with that transformer.

https://taweber.powweb.com/store/PTGPsch.jpg

[Bruce / Mission Amps]

Looks like blue w/red is a 410V tap and red is 0V, or else 690V. OP doesn't say what voltages he has. Lots of options with that transformer.

https://taweber.powweb.com/store/PTGPsch.jpg

Ah.. thanks for the heads up ... I should have looked at the Weber website.
Well, the old tweed amps are fairly simple so it shouldn't be hard to figure this out.

Jetav8r,
If the power tubes cathode bypass cap blew.. it was underrated for voltage, it was soldered in backwards as mentioned or the actual cathode biasing resistor is not grounded.
Leave it out for now as it is not needed for trouble shooting.
Let's hear about some DC voltages without the power tubes installed.

[Jetav8r]

I've posted the schematics from the designer and more pics. I noticed looking at the schematic where my cap blew its shows no ground connection but one was put in, could this have caused it's demise? Also my output tranny is a fender, model #041318, not a weber as previously posted. I need to buy a good digital multimeter, my cheap analog one just won't cut it. However, here're some readings I took using it anyways. With the power switch on, stby off, no tubes installed my AC power tests indicated roughly:

Blue/Red wire coming from PT: ~450vac
Red wire coming from PT: ~690vac

Power tubes pins 2 & 7: ~3.2vac
Pre amp tubes pins 4,5 & 9: ~3.2vac
These should equal since they're chained together.

DC Volts at stby switch coming from SS bridge rectifier: ~650vdc
Seems pretty high to me, is this right?

Keep in mind my readings were using my cheapo analog multimeter. I am hesitant to turn on the stby switch because of the last result. Before doing so I want to get a better meter. Since I am in the learning phase I'm not sure about the correct sequence of events to follow to properly test dc power readings? I've been online researching as best I can, for now I have to put this aside and attend to my real job for the next 4 days for some annual training. I'll do my best in the next 4 days to respond to the much appreciated and excellent help from everyone. Thank you all, Jetav8r.

[JHow]

Power tubes pins 2 & 7: ~3.2vac
Pre amp tubes pins 4,5 & 9: ~3.2vac

You should have 6.3 VAC measured from pin 2 to pin 7. Something doesn't sound right here. Before worrying about anything else it is important to get the power supply right. You didn't measure from pin 7 to ground, by any chance? That would give you only half of the heater supply, or about 3 V?

DC Volts at stby switch coming from SS bridge rectifier: ~650vdc
Seems pretty high to me, is this right?

Yes it is high, if measured correctly. DC should be measured from the red wire at the top of the bridge (junction of diodes) to the chassis. If you are new at messing with hot circuits, clip your common lead from your meter to the chassis, put one hand in your pocket, use the probe in your other hand, touch only the point you want to measure. Sounds silly, but it works.

If you are still getting 690VDC, I would have a look at the taps used on the PT. With bridge, you should only be using two secondaries. It looks like your original designer was shooting for the 690 (red) and 410 tap (blue/red), which should give you the difference (280 VAC) going to the bridge. With tubes in you would probably expect around 350VDC at the top of the rail.

The cap that blew up was a cathode bypass cap. As Bruce noted, you don't need it for now. You can replace it once you have this running. The negative end should have been grounded.

[JHow]

After looking at the schematic (I use the same schematic drawing software too! Pencil 1.0) I don't see much too far out of the ordinary, except maybe the way the tone control is laid out in the drawing is a little odd. Looking at the components, it looks like it should be a basic tweed princeton tone control.

There is a traditional layout for this, which doesn't look like it was followed on the amp. This is probably not causing your problems, but I would fix it anyway. Have a look at this one here:

Adam's Amplifiers: Tone Stacks

You can simplify the layout at the above link by running C2 to lug "3" on the vol. pot, which gives two caps hanging on the pot lugs, and only three wires running to the pots (in, out, and ground).


Also, was looking at your pictures. The first 12ax7 seems to be in parallel. It looks like grid pins 7 and 2 are joined with a bit of bare wire, maybe the end of the grid resistor, at any rate, if that bare wire is shorting on the center post of the tube socket, you will pass no signal. Can't tell exactly from picture.

[Jetav8r]

New numbers

JHow,
I did indeed measure wrong , my shiny new dmm measured 6.3 vac at the tube pins and 377 vdc at the bridge.

Just to clarify, in reading the schematic on the link you sent me, the "in" wire comes from 12ax7 #2 pin 2 and the "out" wire goes to my circuit board, the same location I have it now? I'm just not that experienced at reading electrical schematics, I want to be sure before I rewire.

Finally, you are correct in your observation on valve one, the wire jumping to connect pins 2 and 7, is the resistor wire. It does not touch the center pin, but obviously rest close. Is this wired correctly?
Thanks.

[JHow]

...measured 6.3 vac at the tube pins and 377 vdc at the bridge.

6.3 VAC is what you want.

Just to clarify, in reading the schematic on the link you sent me, the "in" wire comes from 12ax7 #2 pin 2 and the "out" wire goes to my circuit board, the same location I have it now?

I don't think it's wired wrong, just was suggesting a way to do it that is neater. "IN" comes from the first big yellow cap on the board (output of preceding stage, "OUT" is a shielded wire going to the grid of the next stage (pin 2 in your example).

EDIT: After looking at the picture again, it does look a little odd. If we ignore the tone control for a minute (it is really an appendage to the cicuit), the basic volume control should have an "in" line that comes in on one of the end lugs, and "out" line, which is the middle lug with shielded wire, and the other outside lug needs to be grounded (very important last detail). Your set up has a lot of orange wires that cross each other making it hard to tell, but if you trace them that is what you should have. You could try setting it up that way to start and add the tone control later by attaching the two caps and a small jumper wire, per the link I sent you.

...the wire jumping to connect pins 2 and 7, is the resistor wire. It does not touch the center pin, but obviously rest close. Is this wired correctly?

Yep. Per the schematic, the first tube is set up with both triodes in parallel, so the two grids, pin 2 and 7 are connected. I just wanted to make sure that that piece wasn't grounding out on that center lug.