Hello all. I recently got a '73 Hiwatt PA50 from the UK (I'm in the US). Overall the amp looks to be in good shape. Not abused. It even still retains its original capacitor cans (which all show tell tale bubbles on the ends so I'll be replacing those first). I wouldn't be surprised if it hasn't been used much since the '80s. Lots of dust inside. It does have a tech inspection sticker from '99, so someone showed it some love then.
From looking it over closely (and comparing it to another PA 50 I found an old Reverb listing of), I think there are 5 changes from when it left the factory (or garage?) of Mr. Reeves.
Here are some pics to give you a feel for the amp:
And an overall shot of the inside:
I'll post more detailed photos and questions below as I don't know if there is a text or photo limit per post or not.
Small capacitor change (the black Jackson one). My amp:
Comparison amp (blue capacitor):
Besides the change of capacitors, it looks like the original is 100uf and the replacement in my amp is 10uf. Would that change affect the sound/tone any? If so, any ideas how it might affect it?
In the same general area, on the EL34 sockets, some resistors were added (1 per socket). Any 8dea why these were added? To help more modern tubes not overheat? Something else?
Also I am having difficulty reading the 5 band codes on these and hoping someone can enlighten me.
A couple of pics of my amp:
And the reference amp, it's just a solid wire to ground:
Here's a close up of one of the resistors in my amp since I can't zoom in on the other pics. Hope it helps to clarify the colors of the bands.
Last edited by ronin on Sun Jun 01, 2025 5:41 am, edited 1 time in total.
I didn't actually take any specific pics of, just a change to the jack for the power cord, from the original round bulgin connector to the current IEC version.
ronin wrote: ↑Sat May 31, 2025 8:13 pm
If I am reading correctly, both are 22k resistors. Please correct me if I am wrong.
Wrong. red/red/red = 2,200Ω. red/red/orange is 22,000Ω
I apologize, I was hoping the pictures I posted would be larger, but they are not and hence a bit difficult to see. I zoomed in on the reference amp resistor and I thought it looked red/red/orange, but let me know if you disagree.
The resistor next to the cap is 22K. And the resistor to the left of the 22K is 2.2K. They are not both 22Ks. Maybe I'm misunderstanding what you said? Are these the two resistors you are talking about?
You do not have the required permissions to view the files attached to this post.
That's my fault, I don't think I was very clear in my initial explanation. All of my comparisons are between my amp and the reference amp, and my focus is on what's different (what's not original) about my amp. Partially I am curious if any of the non-original parts on my amp are affecting the tone or not.
I hope that makes more sense. Again, sorry for the poor communication on my part.
I doubt there's any sonic change by replacing the original 22K with a different brand 22K. A schematic would show exactly where that resistor is in the circuit. Heck, it may not even be in the signal path.
No worries, I didn't do a good job explaining in the beginning. I appreciate you trying to help me out. And thanks for the feedback on change #4 (the 22k resistor). By chance do you have any thoughts on the other changes, especially #1 and 2?
#1. I doubt there is a sonic difference. Probably measure a bit more ripple on the bias voltage in your amp. Hiwatt liked to use 100µF bias caps in all their amps. I suggest you change the 10µF in your amp to 100µF just because Hiwatt was fond of 100µF.
#2. The added resistors on your power tube sockets are used to simplify bias checks. They're a good thing. No sonic impact. I would leave them.
3#. Both caps are .022µF. I would leave it. I say no sonic impact. Others will say differently.
