Pentode Phase Driver Question

[SilverFox]

I'm getting ready for another build. Perhaps my 4th and still in the learning phase. So far no shocking results or smoke. I have a couple questions before I settle on anything.

One possibility rather than using the Fender AT7-6L6 output design is to try this from a Univox 1221 circuit. I played through this amp for a while and while it was okay it was not the high gain style amp I play most.

In the file attached (Univox 1221) the output section employs the 6AN8 (pentode-Triode) as part of the driver circuit.

What are the advantages disadvantages of this circuit over the usual design using the 12AT7- pi ? Tone differences?

What would the difference be if the output tubes were operated at say 430 instead of 600ish as shown?

Would a Morph circuit work here? What would that change by Morphing pentode to triode?

Thanks you for advice or comments offered,

Silverfox.

[cbass]

If I'm looking at it right it looks like a pentode driving a cathodyne PI.
Never tried that setup but i have compared fender style cathdyne to the ltpi.om another univox amp I prefer a LTPI
Unixox has other amps that ar e similar but use a 12àx7 cathodyne

[matt h]

(deleted)

[Jack Hester]

My Oliver G-300 uses a 6AN8A in the configuration discussed. A really nice amp. As I could find no schematic for it, I made my own and put it in a CAD drawing.

Jack

[sluckey]

All the old Sunns used that circuit. Several old Ampegs did too, although Ampeg favored the 7199.

[SilverFox]

My Oliver G-300 uses a 6AN8A in the configuration discussed. A really nice amp. As I could find no schematic for it, I made my own and put it in a CAD drawing.

Jack

While searching for some Orange schematics I have, (somewhere amongst the GB of files) I ran across an Oliver Amplification brochure with various schematics. Unfortunately while several "G" models are listed not the "300".

If anyone is interested:

I just went over to UTube and the sounds of these amps are uhmm Fantastic!

[tictac]

Wow... those Olivers look like cool amps

And with the option of Altec speakers too!

Interesting reverb circuit.... I know this is a shot in the dark but does anyone know what type of reverb unit would be in the Oliver?

With that cathode driven input I'm thinking maybe a medium impedance tank 600 to 800 ohms?

TT

[teemuk]

The design was largely popularized by Dynaco. I think I've seen books like "RDH" outline the basic idea from early on, though.

[martin manning]

In the guitar amp genera the common thread here is Jess Oliver, principle designer at Ampeg, who later left and operated under his own name. Of course everybody used designs from the tube manuals and RDH, and once one designer found something that worked well for a given application, competitors followed suit.

An advantage of the cathodyne inverter is that it is easy to get a balanced PI output. Adding a "morph" control would be an interesting experiment. It would lower the gain and act as a power reduction or MV. What effect it would have tone wise, I can't say.

[Jack Hester]

While searching for some Orange schematics I have, (somewhere amongst the GB of files) I ran across an Oliver Amplification brochure with various schematics. Unfortunately while several "G" models are listed not the "300".

I just went over to UTube and the sounds of these amps are uhmm Fantastic!

I've got the same literature in my library of e-files, and if you look closely for a very small box, you'll see a reference to the 300's. Apparently, built to custom order.

If you will look at my schematic and layout, and note the components that I labeled as C14 and R32, on the 6AN8A tube, the 1M resistor was open (R32), and all signal ended at this tube. I had to remove C14 to get to it, so I replaced both. What a killer amp.

Reverb is strong, but I did not take note of the long spring tank used. It has an additional bottom, to cover the tank, which is mounted in a cutout in the cabinet. A friend of mine has the amp, but when it comes home, I'll see if I can easily remove the tank and get some numbers.

I'm interested in reviving the Reverb in a Silvertone 1474 that I have. The tank (if that is how their configuration was known) is missing. I may have to slip in a JFET as a CF on the front end, to make a conventional tank work. I have an Accutronics 4FB3??? tank that I believe has the correct impedances to work in a tube-driven Reverb. I don't know where I read about that, but the writer said that he was successful in doing so.

Sorry for getting so far off subject. The 6AN8 tube is what got me started.

Jack

[Jack Hester]

In the guitar amp genera the common thread here is Jess Oliver, principle designer at Ampeg, who later left and operated under his own name. Of course everybody used designs from the tube manuals and RDH, and once one designer found something that worked well for a given application, competitors followed suit. An advantage of the cathodyne inverter is that it is easy to get a balanced PI output.

Someone told me that Oliver was a buddy of the Sunn Amp guys, and that he bought his transformers from them (after Ampeg). He (same someone) also pointed out some similarities to his (same someone) Sunn Sceptre, though I don't remember what those were. Seems they shared ideas.

Jack

[Stevem]

The sunn amps under the 200 watt models used the same transformer set as the DYnaco stereo amps that they where based on.

[martin manning]

Lots of tube options available for this pentode/triode combination for output tube or reverb driver uses: 6AN8, 6BL8, 6EA8, 6GH8, 6KD8, or 6U8. All will substitute functionally for the increasingly-rare 7199 used in Dynaco amps, but with a different pinout.

[SilverFox]

Thank you everyone including future offers, for contributions.

I already stripped the chassis and took some shots so I should get a build post started soon. I think I'll give the existing "Cathodyne?" output a shot and build the power amp first with the Morph control as a basic research project.

I'm sure I'll need some assistance... and looking forward to comments and suggestions.

Link to new build to be posted here:

Silverfox.

[martin manning]

Give it a go! It'll be interesting to see how it turns out. If you use a more normal 400-450V on the plates you'll get a more normal 45-50W power output. The pentode/triode tubes I listed above are common TV and radio types, and should be relatively cheap (<$10) and easy to find.