The Amp Garage

Just out of interest, what car boots are you going to in the UK to pick up things like this, and how much did you pay, if you dont mind my asking?

Neil.

Could someone explain what the second half of V4 is doing? (the first half being final gain stage before the PI). I am reasonably new to higher wattage amp building/schematics, and like the hiwatt/sound city sound.

Thanks in advance.

Neil.

When I was at uni a few years ago I was looking for parts cheap, and a friend of mine got an old stereo unit from a junk shop in the south west of UK. The shop owner was so pleased that someone was interested in taking away the 'heavy hunk of junk' that he let him take it away for free. It was chock...

PCollen, I have got some numbers from your suggestions, and all the voltages seem a bit high, but not massively. I am in the UK, and this amp is set to 240v (not 230 V like they were originally shipped to the UK). When in standby, the bias at the 220k junction will adjust from -51.3 to -62.7 and rea...

some weeks ago I started a thread detailing problems with a hot rod deville. turned out the 82k plate resistor on the PI was open. I replaced this, and the amp worked, briefly. Now there is low volume and all notes are distorted. I have taken the amp to work to get one of the engineers here to have ...

Thing is, the C- test point is giving a good -54 volts. I will still have a good look at the bias bits of the circuit. Continuity was good on the bits i checked, but pressure from the probes when checking the circuit when it is not switched on can easily close up a bad solder joint. Good job I'm not...

Thanks for the tip Billyz. I will have a look at the big caps, I tested them on the board, and they are all functioning, but will look at the joints. The caps have never been replaced, and my friend who owns the amp wanted to know if 16yr old caps are a candidate for replacement. I told him that 16y...

This really is confusing me. I went back to the beginning again. Replaaced the PI resistors again with brand new 1/2 watt's, and followed continuity on as much as I could. Re flowed lots of the pads that looked a bit dry. Checked values on components in the PI. tried a different 12ax7 in the PI afte...

I must have mis-read the voltage as 34V not 94V (I have an A4 printout, so it is a bit or a struggle to read some of it). I did think it odd that it stated such a low voltage on the plates of the output tubes. Low volume and flabby distortion does sound a lot like PI problems to me. I will check out...

I was referencing some of the voltages, and the top and bottom of the OT read at about 480V, not the 34V that the schematic states!!! This cant be right! Do i have a blown output transformer, because that sounds expensive. I will pull the contacts and read the resistances across the OT. I guess that...

My understanding was that these fets were used to bring in different bypass caps on the 3rd (drive) stage and 4th stage (by bypassing the large value resistors) I should get a chance to look at this again in the morning, and have a good look for bad joints. I think i remember seeing the a pad on one...

Apologies for resurrecting this thread again, but I need some more help. I resoldered quite a few pads and replaced a couple of parts (PI resistors), and on testing the amp (with PCB hanging out) it worked as normal. After fixing the PCB's back into the chassis, and tightening up the front panel, th...

Well, I finally got back underneath it, and had a good root around. Some of the solder pads looked less than desirable, so I touched up all the ones that looked bad. I then replaced the other PI plate resistor, as these are known to go also, re soldered the 5W resistors in place, resoldered and chec...

I was planning to re bias it and change the v1 cathode cap at least after i got it running better. I will check out the solder joints you mentioned and ask him to consider a change to switchcraft style jacks. It has that style on the speaker out, but not the input. I guess that is sub-assemblies for...

After looking around on the interwebs, it looks like these PI plate resistors are a known problem in this series of Fenders. I will tell my friend it would be a good idea to source new 82k and 100k resistors at 1W, preferable carbon comp or other high voltage type, and we will swap them all over whe...