New to forum and hobby - resources?

[jy999]

Hello! I'm experienced with electronics, but mostly the low-voltage, digital sort, with a bit of house wiring and such mixed in. As a daily guitar player I've been thinking of building my own amp for a while.. I've been reading Valve Wizards' book, the tube theory seems all pretty reasonable, and planning on saving up for a kit.

In the meantime, I just came across a busted Egnator Tweaker 15 - it was blowing fuses, and apparently a tech said the transformer is shorted. Bought it for cheap, so now I'm going to try and repair it! Thanks to this forum, I just found a schematic, and even a reasonable replacement transformer by ClassicTone, if indeed that's it.

So my question is this- does anyone have recommended resources to get started? Like a solid book or collection of sites, etc.

This is my first foyer into high voltage (except household stuff) and I'm being super careful. I have a new Fluke tester, and managed to secure a differential probe for my scope. I'm looking at variacs now (will probably start with a cheap one from Amazon), and planning on building a incandescent bulb current limiter. I also need to build a capacitor discharger.

First I'll check / replace the fuses, pull the tubes, and ramp up the voltage. If it seems to be shorted, I'll disconnect the transformer completely and confirm..

Any resources would be greatly appreciated!!

Jim

[mac dillard]

https://www.premierguitar.com/articl...ormer-tester-1

[52 Bill]

Welcome to the place.

Back in the day, there was a book written by Jack Darr that detailed a pretty logical approach to amp repair and troubleshooting.
I know that somewhere on this site the troubleshooting text was linked, so you might try reading that.

Build the limiter first and then do the tests on the transformer, it will save you a lot of fuses.

[vintagekiki]

... there was a book written by Jack Darr ...

http://guitar-gear.ru/forum/index.php?app=core&module=attach&section=attach&attach_i d=14475
The Electric Guitar Amplifier Handbook - Jack Darr

[vintagekiki]

So my question is this- does anyone have recommended resources to get started? Like a solid book or collection of sites, etc.

https://music-electronics-forum.com/forum/schematic-requests/25212-links-to-books
Links to Books

[jy999]

Awesome, thanks everyone, this is a huge help. I tried to "like" the posts but couldn't find the button. Maybe it's a limitation as a new user/account.

[jy999]

Follow up - finally got all my safety stuff ready, and I cracked it open today (I never tried to turn it on). Turns out that the primary windings on the power transformer were already cut, while the secondaries were in place. I decided to finish cutting it out (leaving nubs behind to see where it was soldered), and test it on my bench.

I haven't plugged it in yet, but I poured over the schematic and used my ohm-meter. Nothing strikes me as off /unpowered/; tomorrow I'll hook it up to my variac, and run it at 12v to see what the windings give me. However, one thing struck me as strange.

On the secondary side, I have a pair of reds and greens, assumed to be HT and heater (confirmed with ohmmeter that greens have much lower resistance). However, there's also a pair of black wires on the secondary side. This confused me.

I checked where they were wired on the board, and double checked the schematic (https://el34world.com/charts/Schemat...tweaker_15.pdf). One of the black wires was on QC3 on the pre amp board, marked on the board as GRN/WH FIL C/T (on the schematic it says 45VDC). The other was on QC10 (pre amp board), marked as HT CT RED/YEL on the schemtic, unmarked on the board. Both have a hand-written BLK next to them on the board.

This strikes me as really strange. I grabbed my ohmmeter, and indeed, the black pair shows no continuity. I noticed that some pairs did give me resistance readings, so I stripped back the shrink wrap around the wires, and I could see indeed that those two indistinguishable black wires come out at different spots. One comes out between the red wires (a possible centre tap) and the other between the green wires. My ohm meter showed me that these do seem like center taps:
- black#1, CT to green: .4 and .6 ohms to either side.
- black#2, CT to red: 6 ohms to one side, 46 to another

Keeping in mind a tech said the transformer is gone, that may explain the asymmetry.
However, it strikes me as exceedingly strange that a current, modern power transformer would have these unmarked wires going to very different locations.

Tomorrow I'll finish building my bulb current limiter and crack open the variac and see what it gives me.

[jy999]

A-ha. I figured out the asymmetric resistance. In looking at the potential replacement transformer, I realized I can tap the fil winding at 6.3V or 5V.

[vintagekiki]

A-ha. I figured out the asymmetric resistance. In looking at the potential replacement transformer, I realized I can tap the fil winding at 6.3V or 5V.

Winding 6.3VAC is for heat all tubes in amp.
Winding 5VAC is for heat rectifier tube.
Attention. When standby sw is in the on position, winding 5VAC is under HV voltage +400VDC

[jy999]

Thanks for the warning. I did notice that the SB switch has a 360VDC level, but the tap on the green winding (I assume and will test at 5v) notes 45VDC on the schematic? Would you mind clarifying briefly what you mean by S2 (is it standby?), and how you determined the 400VDC? Or at least pointing me in the right direction

[vintagekiki]

Usually in schematics, switch S1 indicates power sw, and S2 standby sw.
Since no voltages are entered on the schematic for TWEAKER HEAD AND 112 SCH, it is to be assumed that for 6V6 +HV (anode) voltage between + 380 - 420VDC

[Helmholtz]

HT is 360VDC as shown in the schematic. I don't see a use for a 5V heater winding as there is no tube rectifier. All amp tubes need 6.3VAC. The 45VDC is derived from HT by voltage dividing.
It is possible that the amp uses elevated heaters meaning that the 6.3VAC is riding on 45VDC.

The 6 Ohm from HT CT to one of the red wires is suspicious, could indicate a winding short.

Let's see what happens on the bulb limiter.

[vintagekiki]

Now it's a different story. Here is some data for PT.

https://music-electronics-forum.com/filedata/fetch?id=852520&d=1545612873

https://www.mercurymagnetics.com/2018/wp-content/uploads/shop/specsheets/EG-TWK15-P-U.pdf

http://www.classictone.net/40-18035.pdf

[g1]

In looking at the potential replacement transformer, I realized I can tap the fil winding at 6.3V or 5V.

That is true only for the classic-tone that VK linked in post #13. That transformer also has another dedicated 6.3V winding. The Egnator transformer would not necessarily be the same.

If your readings for the HV winding are correct (6 ohms one half, 46 ohms other half), then I would agree with the tech that said the power transformer is bad.

[Helmholtz]

BTW, a 5V winding for a tube rectifier can never be a part/tap of the 6.3V heater supply.