Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Throstur on December 19, 2010, 02:39:32 pm
-
I just finished my second 5E3 Replica in one month. The first one was perfect from the moment I flicked the power switch. This one is different in one way. Both Volume pots (Alpha) have to be turned up to 3 before any sound comes through! Any thoughts on what could cause this?
-
SAME pots?
Ohm-meter from ground to wiper, see if it stays zero (or super-low) from "0" to "2.5".
-
SAME pots?
Volume Normal, Volume Bright and Tone are all Alpha 1 MEG. One could be faulty, but the odds that two are faulty in the same way are low, right?
-
I meant: same (brand, batch) as the pots used in your other recent build.
> odds that two are faulty in the same way are low, right?
Got an ohm-meter?
In mass-production, screw-ups are not always noticed quickly. In a few minutes they could have made 2 or 2,000 pots with bad traces.
-
I meant: same (brand, batch) as the pots used in your other recent build.
> odds that two are faulty in the same way are low, right?
Got an ohm-meter?
In mass-production, screw-ups are not always noticed quickly. In a few minutes they could have made 2 or 2,000 pots with bad traces.
English is not my native language so I often don´t understand, or misunderstand , but I try my best.
I could not have built over 20 tube amps and repaired hundreds of them without a good multimeter. I also use a signal generator, oscilloscope, signal tracer, quality capacitance meter and lots of other specialised equipment.
I have to order every little item I use for repair and amp building from abroad. There is almost zero knowledge about tube amps, and no source for parts in my country. I´m just trying to do my best with what I have.
The Hoffman site and this Forum have been the single most important source of information and help in my attempt to learn about how to build and repair tube amplifiers.
-
1) Are the 1 M pots Audio or Linear pots ?
In the other amp the pots were Audio ?
If you used 1 M Linear pots instead of 1 M Audio pots the taper isn't the same
2) Try with different 1 M pots
Kagliostro
-
I have to order every little item I use for repair and amp building from abroad. There is almost zero knowledge about tube amps, and no source for parts in my country. I´m just trying to do my best with what I have.
Ahhh - a fellow sufferer. Welcome to my world.
Dodgy pots could be one source of low volume trouble. A leaky coupling cap (between the driver stage and the cathodyne?) which affects the bias, could be another.
-
Your english is fine.
You asked for "any thoughts".
I thought you should ohm-meter the pots.
You replied with the resonable assertion that two bad pots are unlikely.
I can not know if that means you don't have a meter, or do but didn't use it, or did use it and the pots seem fine.
Maybe my english could be better.
I did not mean to insult you.
I hope you find your problem.
-
Sorry PRR if I have offended you in any way. You have helped me many times over the years with your good advice and vast experience. I don't have access to a computer where I do my amp work, and sometimes get carried away speculating when I get home where my computer is. PRR, please accept my apologies.
-
At last I got around to address this problem properly. Here are my findings:
The pots in my 5E3 came from batch of 30 Alpha 1Meg pots that I bought recently.
First of all I measured the pots in the 5E3. They all measured from 1,5K to around 940K which is no good in this kind of an amplifier where the volume pots play major role in blending tone. The pots do not start to react until bout 2.5 -3
Next I tested the 27 remaining pots and the outcome was to put it mildly, not good. The lowest I could get on any of them was 0,5K and most of them where from 1K to 1,5K at the low end. At the high end I measured from 850K to 950K. I have until now been under the impression that Alpha makes products. This batch of pots seems to indicate the opposite.
It would be nice to have your opinion on this. Are there better pots out there?
-
I believe 10% is the tolerance. IMO, A 1m pot reading 950k is not going to make a difference. I use Alpha's all the time.
-
I believe 10% is the tolerance. IMO, A 1m pot reading 950k is not going to make a difference. I use Alpha's all the time.
I have used Alpha on all my builds and have until now been satisfied. The high end reading does not matter. The problem in this case is the low end. 1,5K means that the pot doesn´t start to react until 2.5 or 3 and then kicks in at considerable volume like some cheap production amp.
Maybe I just got a bad batch as PRR suggests.
-
I've worked in manufacturing plants my whole career and have watched Parts that did not pass quality control and were pulled off the line get re inspected and zoom off they go into the world.
It's amazing how lax tolerances can be when an order needs filled today by 3:00 pm when the shipping truck arrives.
It used to make me laugh when I would see half million dollar fixturing jigs with layers of masking tape put on locating datums so that when a part was placed on them a GO/NoGO light would go green and down the line it would go.