My second Tubelab Simple PP! A pentoad Frog!

[mctavish]

Working on my second simple pp! The first one was set up in triode mode and this one will have pentode outputs with a regulated screen supply.

I am thinking a pair of stacked glowtubes would suffice for a screen supply and look pretty...

The outputs are a pair from stancor out of a Heathkit AA-100 that are 7kpp with 0-4-8-16 secondaries.

Its coming along...

 

[mctavish]

pics

 

[OldHector]

Being ignorant here, but what are the black sockets above the speaker terminals? I had assumed fuses.
If they are not fuses, does anyone bother to protect their speakers? I have 5 fuses in My SPP, 1 on the live in, 2 on the seondaries, and both speaker positives.

 

[mctavish]

They are the neutrik speaker connectors. I love them! It’s pretty much all that I use these days. I just put binding posts posts on there, in case I have some random speakers I want to try out, they don’t have my cables on them already. I only fuse the primary. In my opinion there’s no need to fuse the speaker outputs, you already have galvanic isolation because of the output transformer. If the secondary of the output transformer is properly grounded, there’s nothing really that can happen. I’ll post some more pictures today when I get back home, right now I’m doing a tour of the new train tunnel being built in Semmering, Austria

 

[mctavish]

Made some progress!

I have tested everything out and it sounds great!

B+ is around 325, which is about perfect. I have the screens connected to the film caps at the moment.

I have thought about a pair of 0D3 tubes in series for a stabilised screen supply, or an 0D3 and 0B3 for 240 volts.

How much difference would a regulated screen supply make in this amp?

Would 300v be better that 240v for the screens or is 240v a safer choice?

I have not made many pentode amps, I know that a steady, stiff supply is important for the screens in pure pentode mode but is it worth the effort to make a stabilised supply?

 

[mctavish]

more pics

 

[mctavish]

George, how much feedback (db) does the amp have with the stock values in the schematic?

Is it enough for straight pentode mode or could I lower the FB resistor to say 4k or so?

I unfortunately dont have my scope any more so I dont have a way to check the squarewave.....

I am hoping that with the gridstoppers and the layout that I dont have any oscillations.

Is there a good way (or ANY way) to check for stability without a scope?

I have snubbers on the secondary of the output transformers, and I have them wired like they were in the original amp (Heathkit AA-100) with the 4-ohm tap grounded and the feedback taken from the 0-ohm tap.

 

[mctavish]

Well I have decided to go for it!

I am punching the holes for the 2 octal sockets tonight and will try the Gas tube regulation for the output screens.

So I will basically end up with 345v on the plates and either 300v or 240v on the screens.

The amp sounds great as it is but I thinke a lower, regulated voltage for the screens will contribute to a longer, happier life for the output tubes....

I am still wondering about the feedback ratio though....Anyone know the db amount with the stock values? George?

 

[mctavish]

Well I have installed the 2 gas tubes. The b+ is now 365 and the screens at 300v.

I have changed the feedback resistor to 4k7 and the things sounds amaying!

The impression is that it has gobs of power and it has no trouble driving my somewhat low sensitivity SEAS L17 speakers.

Also sounds great on my Fostex FF225WK 2-ways with neoCD3 ribbon tweeters.

Thanks for a great design George!

 

[Tubelab_com]

George, how much feedback (db) does the amp have with the stock values in the schematic?.....Anyone know the db amount with the stock values? George?

The feedback varies greatly with the OPT being used. Unfortunately much of the information I had on the SPP was lost when my computers and other stuff (including the original TSE amp) was flooded during a hurricane. This happened during the development of the SPP, and is why some of the build information is still missing.

I remember a value of about 8 db sounded the best to me with my speakers, but I was using 6600 ohm OPT's because I had, and still have lots of them.

Is there a good way (or ANY way) to check for stability without a scope?

Unless the OPT is really bad, and mine were repurposed guitar amp transformers, the amp will be stable. Lacking a scope the capacitor value in the feedback path can be tweaked by tuning for the best sound on signals with broad high frequency content like cymbals. Tune for the most natural sound. You are trying to balance the high frequency response with the rest of the amp.

 

[mctavish]

Thanks for the reply. At the moment I have no cap in the feedback circuit and it sounds quite good with high frequency content.

I will start out with 68pf and go from there, unfortunately I will have to wait until next week, we are moving house this week....

The outputs are of very good quality, I have used them before and I can say with confidence that they were the BEST part of the Heathkit AA-100...

One other thing, I have only .86v at the cathode of the input triode of the 12AT7s. Is this normal or am I looking for more like 1-2 volts there?

 

[mctavish]

Got everything up and running with the gas reg tubes. sounds fantastic and all of the voltages check out now.

I am still curious about the low cathode voltage on the input triode - 12AT7.

Is this normal and a part of the design or should I be looking for more negative voltage on the cathode.

I am now thinking about playing with a bit more negative feedback as the gain seems still to be a bit too high in pure pentoad mode....

 

[baggerbole]

What was your first tubelab build?
And how does it compete to the SPP?