Vintage amp restoration and Beyond

[GoldenVulture]

The squealing at high volume has slowly disappeared almost entirely.
I'm wondering whether one of the KT88's isn't up to scratch. After a while sometimes, one particular one of them will start redplating. I changed it's position twice but it's still doing it some times. I might have to get another one to replace it. This amp is running pretty close to maximum voltages for the KT's.

A neighbour dropped in before. I pumped out a few chords at full volume to see if he thought it was loud.
YES ! :wwe: Loud and Clear . He commented straight away that the bottom end was really clear. :toast:

It sounds pretty good at low volumes too. I still need to try and cut down the microphonic stuff happening at V1.

I'll try and get some sound clips together soon.

[Securb]

I'll try and get some sound clips together soon.

You don't post clips!

[GoldenVulture]

You don't post clips!

Yes I do ! Old ones ... :evil:

[treyhaislip]

Savage lol

[GoldenVulture]

The squealing at high volume has slowly disappeared almost entirely.
I'm wondering whether one of the KT88's isn't up to scratch. After a while sometimes, one particular one of them will start redplating. I changed it's position twice but it's still doing it some times. I might have to get another one to replace it. This amp is running pretty close to maximum voltages for the KT's.

V1 which was squealing is almost squeal free. I discovered it squeals like a pig if you don't have a cover on it.

I finally nailed my red plating and my stupidity. I thought the bias voltages it was running seemed a bit low. I should've thought of this sooner
 As there were no reference voltages on the circuit I've been using, I checked what my other amp that runs KT88's is biased at. Yes they were too low. The higher the plate voltages the higher the bias needed. As the Power Supply is a hack of two circuits I had to modify it again by changing a couple of feed resistors. That put the bias into a usable range. No more red plating.
 At the current time I'm running it at - 42 Volts. I tried it at - 46 Volts for a while, I might go back to it. I'm looking for the bias sweet spot. The 88 data sheet suggests bias voltages in the - 40 and - 50 volt range depending on your circuit confiuration. This circuit seems to work in the 40 volt range.

Looking at some of my pics of the power supply I noticed another wiring mistake. None the less it has been rectified.

[GoldenVulture]

I received some new valves today. I got a pair of Gold Lion 12AX7's and a pair of Mullards [ Both re-issues ] Tried the GL's in V1 and V2 and a Mullard in the PI. A mild improvement over the old EC83's. Swapped V1 for a Mullard. I think it's better there than the GL. Also, I got rid of the squeal completely. It was a matter of swapping the Output Transformer leads [ Phase issue/ negative feedback loop ] I remembered encountering the issue many years back.

:toast:
Hopefully I will get a sound clip together soon. [ It's only been almost a month since I last said that. ]

[GoldenVulture]

I had the Mid and Bass controls around the wrong way so I fixed that as well.
[img][/img]

[GoldenVulture]

Also, I got rid of the squeal completely. It was a matter of swapping the Output Transformer leads [ Phase issue/ negative feedback loop ] I remembered encountering the issue many years back.

What a comedy of errors. It turned out I hadn't got rid of the squeal. It moved from the max end of the controls to the minimum end.  Then I noticed another wiring mistake on V1. I was inputting into the wrong pin, into the cathode and not the grid pin. I fixed that and changed the OT wires back how they were. The squeal is now gone except for a small trace of it when everything is maxed out and nothing plugged in. Plug in a guitar and  you can get feed back pretty easily because the pre-amp has a mad amount of gain.
It doesn't sound quite as clean wired correctly  but I may modify the pre-amp and wind back the gain factor.
Anyway it appears the squeal was because of my incorrect wiring.

I've started wiring in another pre-amp section for a second channel. This one is based on the pre-amp circuitry of my Vase amp. It will be interesting to compare the two channels when I've got it done.
 Then an FX send and return to be made.

[Securb]

Is there a way to wire the pre-amps to cascade on each other in one channel or be run separately? I would imagine you could get an unholy amount of gain.,

[GoldenVulture]

Is there a way to wire the pre-amps to cascade on each other in one channel or be run separately? I would imagine you could get an unholy amount of gain.,

Yes you could but the gain would be ridiculous and probably feedback internally. Essentially a lot of pre-amps are cascaded. 12AX7's and that range of valves ,AT7, AU7 are double valves. The source signal goes through one half and then the other and if you have more than one in the pre-amp  it will once again go through one and then the other half of the valve.  Pretty much what happens in this build.
The amount of gain on this is already extreme to the point of compromising the tonal qualities of the pre-amp, to me.