Anyone built a Single Ended EL34 Hot Cat?

[Shawnee]

Hello everyone, first post here. I'm an electronics tech by trade but I am no engineer. I've done some amp tweaks but never built from scratch. I have been agonizing over my first build and am curious to see if anyone has built a single ended EL34 Hot Cat. Probably wouldn't stay as clean as a Hot Cat 15 in push/pull but might sing a little more over driven. I have the old 2005 schematic so I thought that I might build the preamp and attempt it. Any potential issues? Any insight that could be offered would be most helpful!

[sluckey]

I'm not familiar with that amp. Could you post a schematic?

[Shawnee]

It's a Bad Cat amp. The Hot Cat 30 is a push/pull cathode biased EL34 amp that puts out 30 watts. They have a Hot Cat 15 that has a pair of EL84's. I was wondering if anyone had built the preamp into a single ended EL34 amp.

[DummyLoad]

the phase inverter is also the mixer-amp for the clean and gain channels. if you want to keep the preamp tone structure as is, and keep both channels then build the LTPI and just load one side with the output stage. you could use the other side of the phase inveter for a line out.


--pete

[Shawnee]

Thank you for the insight. I understand what you are saying............sort of. When I looked at the schematic, I was wondering how the clean could sound so good with one stage.

[shooter]

single ended EL34 amp.

 so you want just 1 tube, no PI?    I built a SE that can use (sorta) EL34, 6550, or KT88
The 34 was the grungiest, the 6550 ok and the KT was cleanest.  Runs about 16Wrms. 
but it's an all octal, low gain.  there shouldn't be much problem mating the pre section to about any SE big bottle.     

[Shawnee]

Well I have to admit that I like the idea of building a single ended EL34 Hot Cat but have no idea how to do it!
I know that you don't have to have a PI in single ended but this single stage clean channel could sure use another stage of some sort. In an earlier post Dummyload suggested a "LTPI" that I am assuming means long tail driver to fix that. The dirty channel has what looks like 1 stage plus 2 cathode followers?

[Willabe]

The dirty channel has what looks like 1 stage plus 2 cathode followers?

The dirty channel has 3 gain stages and 1 dc coupled cathode follower(CF).

That's a lot of gain in front of a single ended(SE) power amp. But it does have 3 volume controls, gain/drive/master volume(MV) to tame it down. 

 

[DummyLoad]

if'n ya wanna push the gain through the roof, bypass the 1k of v3b with a 1uF. make it switchable.    


--pete

[PRR]

I was wondering how the clean could sound so good with one stage.

You need *two* triode stages to get from guitar to power tube.

Here there is preamp and the power-stage driver (/inverter), two stages.

You can cut to one power tube, but you still need something for "V4".

The logical thing is to keep just V4A, short the 47K under it.

However V4A V4B also mix the clean and dirty. If you use a passive resistor mix, you lose more gain than a 2-triode channel can stand.

Some topologies just come out better than they deserve.

I suggest you use a switch at V4A grid (F) to select clean or dirty.

[kagliostro]

keep both channels then build the LTPI and just load one side with the output stage. you could use the other side of the phase inveter for a line out.

I find this thing very interesting

There are disadvantages/contraindications using this way ?

Thanks

Franco

[Shawnee]

Thank you for the replies!
I definitely want to put a loop in it since I run a rack unit for reverb/delay etc.
Another great thing about a loop instead of just a line out is that you can put a dummy load on one amp and go out of the send into the return on another amp. That way you can mix and match preamps and power amps without having to build everything just to try it!

Another thought on this build is maybe using an EF86 as the clean preamp tube. Then I wouldn't need a PI at all and I could use that tube for a loop...........

[macula56]

I have often wondered, what is the purpose of having no cathode resistor on V1a? or is that a mistake in the schematic? anybody know?

[sluckey]

V1A is a called grid leak bias circuit, instead of the more common cathode biased circuit. Grid leak bias usually requires a cap coupled input and a very big grid leak resistor. I believe the 1M and 5.1M resistor labels are mistakenly swapped.

[PRR]

> I believe the 1M and 5.1M resistor labels are mistakenly swapped.

Ya never know on a gitar amp named Bad-Hot, 'specially on the "dirt" path..... but I agree. Conventionally the grid returns through 5 or 10 megs, so the small grid current makes a not-small grid voltage. 1 meg will risk distortion on medium-strong signals (which just could be the intent).

[John]

What are the advantages/disadvantages  of the grid leak bias type?