Gulsekt-88!

[matt h]

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[John_P_WI]

Yep, understood well.

[Meat&Beer]

I suspect it will make yeti cry.

[David Root]

Woooo! That is wonderful! Completely different than my current SE KT88 planned build.

Also matth advising me, he is the Guru of Ghetto UL!

Here's my draft schem. It is not yet complete as I am going to add a 12AX7/5751 in parallel in front of V1A pentode, for proper preamp overdrive in that pentode, and a buffered loop also probably a 5751 or maybe 12AT7.

The PS may have to be split with the two extra tubes as I'll be pushing the HT and heater supply a tad hard otherwise. Would put in a 270BX PT and a 157G choke (both in stock!) and run all three pres and the driver 6GH8A.

[matt h]

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[David Root]

Well I was going to use the 5V winding for the relay PS but I think I have a little 5V or 6V 50mA or so PT I could use for that.

That would have to have a voltage doubler in it too if I use the 5V winding.

And in the back of my mind is also this question--what if the TV tubes' heaters hum a bit, I might have to go to DC on them, so why not just go DC on all four?

[matt h]

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[David Root]

OK, a Greinacher voltage doubler then. On the first stage 12AX7 and the active loop. I have a couple of 10,000uF 6.3V caps which should do nicely. Unregulated but maybe a 6mH 2A choke to help smoothing a bit?

And elevate all other heaters about 50V or so.

[matt h]

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[Meat&Beer]

What's the h-k spec for the tubes?...

Sounds like a quote from Predator..

Dave, that looks great! Double the GUL fun, right?

Here's a little update on this slow moving monolith of molten metal.

Try as you may to place a hole the correct size and location... Minor issues. That's why this is a proto! Coming along very well I must say. I can see the rebuilt having about a 1/2"-3/4" shift of the front controls in respect to what's behind them so far.

We'll see, still a bit to go before relocation ideas really coagulate. The clipping rotary is going to be insanity! (For test purposes)

Question, I couldn't see putting a 600v.+ .1 cap on the first grid. So, instead I put this 10v. ceramic in there. Other than being old and ceramic (it tests right) should I, and why, put a poly something or other in there instead? I just couldn't force myself to install a cap rated for 630 volts when I know there are not nearly that many at that point... Size restraints considered, also.

This cap sees the output voltage of the guitar pickup, in essence, and after the 27k even... Am I right?

[David Root]

matt, 6U8 and 6GH8 are both 100V DC, heater +ve wrt cathode, 200V DC + Peak, and 200V DC + peak, heater -ve wrt cathode, no DC only stated. Applies to triode and pentode.

How about 3K3 screens? I think I have some 3W.

M&B, looking good! If that cap is spec'd 600V there may be a good reason for it, but it sounds like it wouldn't need to be.

[matt h]

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[Meat&Beer]

The .1uf isn't stated to be a 600v, what I should have said is that in my parts drawer of .1's, I only have 600v+ caps available. Nothing is telling me what the cap should be rated for. Then I found a 10v and plunked that in there instead. We all know this is a proto, so I'm trying to save more better parts for when I really mean it. Using recycled wire here where I can, things like that.

Hmm... How do I figure out what the K value that the grid will see with the 27k hooked to a .1uf cap? I mean, what's the formula, or how ya figure it? (I tried to cheat using my DMM, but it wouldn't tell me...)

Omitting the .1, the feedback cap would be 3.3pf for a 20kHz roll off. I'll be popping one of those in there, as soon as I can figure out the right value.

As for an external test board of diodes and such, that's a good idea. I've been pondering how I'm going to get all of these tucked in there without destroying them for future applications, as I'm sure I'll be not using the 20 or so different clipping prospects I have available for auditioning. Got time to figure it out.

[matt h]

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[Meat&Beer]

Fired up! A little hum, buzz, and some squeal at high gain settings, but overall sounded pretty nice at initial listen.

There are quite a few things to do still, install 1st stage roll-off cap grid to cathode, Add a cap/move a cap/change a cap etc., diode clipping rotary hasn't even been explored yet, (at least) two other ideas on a switch have to be explored, CF Master is probably going to be relocated to after the tone stack, stuff stuff stuff.

Voltages are good, because of the PT limitations the KT-88 can't be supercharged like they normally are, but so far, so good. I'll post more specs as the end result is more of what it should be, still tweaking obviously.

Plenty of gain on this little machine! Clean tones? Yea... not so much. Not what we were particularly looking for either. But, they make a switch for that, too. (IF I even feel the need, plenty of my other amps to clean very well, I do the heavy stuff mostly anyway.)

I'll post sound clips or a video when things get more or less straightened out with it.