Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Chaopsychic on May 03, 2024, 01:33:47 pm
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Greetings ladies and gentlemen,
this is my first post on your forum so i'll try to keep it short and simple.
Got this old guy on the bench for no sound and a blown fuse (F202).
The amp got a new set of JJ tubes just to make a fresh start as they were never changed.
I've studied the schematic and have checked and changed a couple of things as i noticed around 2.8v on pin9 of the preamp tubes instead of 6ish.
First i replaced both all Bias & (HTR - 14V) supply sections and every 1N4148 i could see.
As people from the computer suggested, i checked all the transistors and relays, took them all out to see if the heaters come back to normal without any luck.
I also replaced C7, C8, C145, C119 and removed D108 & D109 that looked awful.
As i took V1 and V2 out i noticed that heater voltage rose and when pluging a cable at the return jack the amp produced sound.
Plate voltages are in 400ish area so i didn't touch the high voltage section.
What i've come to realise is that there is some crakling noise, no guitar sound and low heater voltage.
I also tried an other power transformer as i have a second valveking which had other issues.(now fixed)
I make no money out of this, this amp is for a bandmate of mine and i'm trying to help him.
The thing is that when i remove a preamp tube (V1, V2 or V3) the heater voltage rises 1-2 volts.
It's just that having all 3 of them hooked up drops heater voltage and causes no sound coming out of the speakers.
Attached you will find the schematic and any help would be much appreciated.
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Check voltages in the heater power supply.
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According to check points in the schematic
(HTR) shows 34V
(14) is 13.3V
so i guess it's alright?
This is the current situation
https://drive.google.com/file/d/19_fWe5NF0LV8aqhpFeSYi1rPnvpScVt9/view?usp=sharing (https://drive.google.com/file/d/19_fWe5NF0LV8aqhpFeSYi1rPnvpScVt9/view?usp=sharing)
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Reseat that ribbon cable on both ends. Any better?
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The ribbon multi cable thing is soldered directly on both pcbs and of course i have tried resoldering it.
Also, moving it aroung doesn't introduce any voltage fluctuations.
I wonder if the tube sockets themselves could cause issues like that (they are branded Ruby) otherwise i do not know what else to check regarding this issue..
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Today i decided to replace the ribbon multi cable with new cables with no luck.
Any help would be much appreciated.
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I have worked on 2 valve kings. 1 50 watt, and 1 100 watt. When I got the 100 watt, it had no heaters at all.
After I opened it up, I saw that one of the on board fuses had blown. Seeing that it has 3 on board fuses, I chose to purchase plastic fuse holders, to make those easy to change in the future. I got that part from ebay, easy to find on there, they use physically smaller fuses, but the pins on the fuse holders match the spacing on the board where the stock fuses are. I saw burning near the rectifier diodes and also noticed they had used the smallest little diodes for the 3 different rectifiers. I got some beefier diodes, and went ahead and replaced them all.
Doing that got the amp running, but there was crackling, and no reverb.
I measured bad readings at the 4558 op amp reverb driver chip, so I pulled that, and installed an 8 pin socket there, and new chip into that socket.
As for the crackling, I replaced the preamp tube anode resistors.
I believe I changed out all of the power electrolytic capacitors, and 1 or 2 power resistors that showed heat issues. I remember the long ceramic one, because I had to order it and it's a weird value.
Those component changes fixed the 100 watt amp.
The 50 watt that I worked on only had crackling problems. I replaced the preamp tube anode resistors and that fixed that. I will say, on the 50 watt, I went ahead and changed the power electrolytic caps, added beefier diodes, added fuse holders, and gave it a bias pot for the fixed bias output. I still have the 50 watt, and I really like that amp.
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https://www.ebay.com/itm/255127533442?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=XqJRrwEERWC&sssrc=4429486&ssuid=0kRPe7NCTWy&var=&widget_ver=artemis&media=COPY
That is not exactly what I bought, but I believe that would fit the hole spacing. Very similar to what I bought
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This is the one I bought
https://www.ebay.com/itm/251128282676?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=Cio5LutuRtG&sssrc=4429486&ssuid=0kRPe7NCTWy&var=&widget_ver=artemis&media=COPY