Introducing Myself

For those building and repairing amps on their own. Learna and share ideas.

Introducing Myself

Postby ZombyGuitarist on Thu Aug 18, 2016 6:54 am

Hey there,
I currently do sales and guitar work at a local store. I'm in my early 20's and i'm really wanting to get into amp repair. I currently have 2 amps the are in the back of the shop as kind of my practice amps. A 1965 Fender Champ, and an Epiphone Valve Senior. Now i have a group that is helping me learn, but i'm not all that sure some of the things i am"taught". I don't have my schematics and my readings in front of me right now, i will have to post them later. But i am curious. On the diagram after the power transformer, will the voltage be near that of the schematic (if so what percentage) or will it be exactly that. Any input is greatly appreciated.
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Re: Introducing Myself

Postby gusfinley on Mon Aug 22, 2016 3:11 am

No, it won't be exact. Tube amps are more of an art than a science in regards to tolerances.

Since the high voltage power supply on a tube amp is unregulated, you can never expect the voltage on an amp to be exactly what is listed on the schematic.

Every power transformer will be wound a little differently, and everyone's mains line voltages will be a little different also. The ratio of secondary to primary windings determines the output of the power transformer, so it will make small differences in line voltages larger differences at the secondary of the power transformer. I have seen the Plate voltages on some amps vary as much as 25VDC on a single day!
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Re: Introducing Myself

Postby ZombyGuitarist on Mon Aug 22, 2016 4:25 am

Thanks for the reply!
What i am about to post is something i posted in a different part of the forum. I haven't gotten a reply on it yet. Note that i am copying and pasting, so there may be something i already asked. This just goes into more detail.

1) 1965 Fender Vibro Champ
Schem at: ... _schem.gif

Problem: Fizzes when tubes get hot.
Diagnostic Measurements:
-On the "red wire" leading to the rectifier, it says its supposed to be reading 315v, but i am reading 372v.
-On the "green wire" leading to the heaters and the pilot light, it says i am to read 6.3v. I'm reading 7.3v.
-After the rectifier, it says i am to be reading 355v. I am reading 492v.

The gentlman that has been mentoring me says its the power transformer. I thought that made sense until he said the same thing about the Epiphone Valve Sr (which is a 2000's model amp vs a 1965 model Vibro Champ). He says that the switch in America from 115v to 120v coming from wall outlets is what is messing it up. That the transformer is meant for 115v. Does it really make that much of a difference or is it something else. Logic would make me think otherwise.

2)Epiphone Valve Senior
Schem at: ... 774__Valve Sr schematicImproved.pdf ... proved.pdf

-Before the solid state rectifier it is supposed to read 300v, it reads 335v.
-At "A" on the schematic it says it supposed to read 395v, it reads 480v.

My questions are:
Is he correct?
Is it something else?
At what percent of the original rating is it okay to go over, and when does it get dangerous for the amp?
Does the "voltage change" effect the out come and if so, is it enough to hurt the amp. (Again, this makes sense to me, but i would like to have input from someone who has been doing this longer.)

Again, any info is very much appreciated!
Last edited by ZombyGuitarist on Tue Aug 23, 2016 7:34 pm, edited 1 time in total.
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Re: Introducing Myself

Postby pdf64 on Mon Aug 22, 2016 4:47 pm

All the secondary etc voltages in an amp are just a ratio of the mains voltage; the mains voltage (at the time of measurement) puts all others into context.
Hence it is beneficial to note the mains voltage when undertaking voltage surveys.

Tube info usually notes that heater voltage limits are nominal +/-10%, or even +/-5%.
So an absolute max of 6.9V across 6.3V heaters.
If you are measuring 7.3V, either your meter is bad or the amp is connected to a mains voltage up to 16% higher than it was intended for.
If the latter proves to be the case, then a mains bucker may be the best way forward, see ... ntvolt.htm

Unfortunately, as well as vintage USA power transformers, many modern worldwide type PTs have this issue, as their dual primary windings are intended for series connection for 230V or parallel connection for 115V.
This results in such PTs putting out excessive voltage when operated in 120V or 240V areas.
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