How about a P-P DHP amp?

[Gary P]

<img src=http://home.pacifier.com/~gpimm/47_top.jpg>
For a amusing diversion I built up a DHP P-P amp using the 47 pentode. The story starts at my favorite local surplus shop. He had bought out a large stash of tubes and in them were many Philco globe 47's still in the box. I picked up 5 at $3.00 each.
The amp is a simple design using a transformer for the phase split duty, a 6SN7 as the differentail driver DC coupled to the 47's. There are no coupling caps in this amp. There is voltage feedback from the plates of the 47's to get the output impedance to a useable number.

The amp puts out close to 5 watts per channel and sounds really nice. Sweet and clean, fast, dynamic, and suprisingly has good bass controll. This was a suprise for me as the damping factor is a whopping "1". This is really a plesant little amp to listen to.

The schematic and underside picture is on my web page in the schematics section.
<a href="http://home.pacifier.com/~gpimm">Gary P's diy page</a>

[SHiFTY]

Nice, globe tubes are very cool...

Directly heated pentodes? Maybe the next big thing?!

[Sch3mat1c]

Try 1614(?) on for size... too bad it's metal. Poop.

Tim

[James D.]

Hi Gary,

Nice amplifier. I've been toying with something similar myself - so thanks for the example and the encouragement it gives.

I notice you have the screen grid at a higher potential than the anode - have you investigated how this affects the distortion characteristic of the pentode? I ask as work I did some years ago showed that pentodes were sensitive to the screen grid voltage being near to the anode voltage and worked best with the screen lower than the anode by about half the p. to p. input voltage swing. By best I mean lowest total distortion and best distortion spectrum. This was for true for THD and IMD. The other factor concerning screen grid setup was to ensure that it's voltage was very well regulated at dc and that it is at ac ground i.e. very well decoupled from the signal. This yielded by far the best measured performance.

ciao

James

[bcherry]

Someone also converted the Joplin 2A3 pp to 47's for an interesting comparison between the tube sets.
http://www.audioasylum.com/forums/DI...sages/525.html
http://www.audioasylum.com/forums/DI...sages/556.html
This is the log of the build up to 2A3 class A. I'll post the pictures of the 47'd version in a day or so.
http://www.diyhifisupply.com/LanceJoplin/lance.htm

I'd like to try this too and I've found some UX5 sockets but still looking for 8 pcs 47. Mostly can only find ones and twos. Anyone know where I could find some?

Brian

[bcherry]

Here's a picture:

[Gary P]

Hi James,

The setup with the screens connected directly to B+ was to get the amp up and see what potential there was. I was really suprised how nice it sounds.

This evening I added a CCS fed shunt reg consisting of a pair of 0A2's stacked. Just couldn't have an amp without a CCS someware in it! Now the screens and driver stage is running from the shunt reg at 300 volts. The plates are at 330 volts and the cathodes are at 100 volts.

This really helped to open up the sound. I can hear much deeper into the music with this setup. It deffinatly has the DH sound.

I have not tried triode connecting them. That seemed the easy way out! This experiment is to see what the pentode connection can sound like with just local voltage feedback, no global feedback. Really nice!

To further the experiment I plan on adding a set of 9 pin mini sockets and another 6.3 volt filament tranny so a set of EL84's can be run. It lookl like the EL84 would be pretty happy at the 47's operating point. Then we can compare DHP sound to IDHP.

Gary

[James D.]

Hi Gary,

You have restored my faith by putting the CCSs in

I really like what you are doing with this. Trying the EL84 will be interesting.

I'm doing some experimenting with 5B-255M (807 electrically), comparing Class A1 triode SE, PP & Class AB1 tetrode SE & PP. So far the triode PP is winning hands down both subjectively and measurement wise - lowest distortion and best distortion spectrum. Surprisingly the tetrode PP is second best and the triode SE third and tetrode SE not very nice... These results were supported by the measured results. I should get my finger out and graph and write up the results - then I can post them...

The Class AB1 tetrode result was a real surprise until I measured the distortion spectrum up to about 20Watts output second harmonic dominates with 3, 4, 5, 6 ,7 etc. running lower with a nice smooth amplitude roll off from the level of the second harmonic. Really a very musically friendly spectrum. above 20 watts the odd orders increase above the even order harmonics and the sound starts to go hard and brittle...

The Class A triode SE has way too much 2nd harmonic - close to 2% at 1 watt! It has quite low third harmonic at 0.15% at almost all power levels. It sounds good at low levels but at higher levels starts to thicken.

The Class A pp has a low level, 0.5% at 4watts, THD and a great distortion spectrum. It's very, very clean and relaxed. I would love to run this off against my 2A3pp but I use the same OPT at present! I have a set of 6B4G arriving and another OPT so I'll run it off against that later this month - the 2A3 are from my sons PSE amp so I can only borrow them for short periods!


I have been toying with the idea of DHPs for a while - your work is really egging me on to do it - Thanks

ciao

James

[bcherry]

Thanks to Lance, the builder of the other 47 amp shown:

"Let's talk about the 47 for a minute. The data sheet says that the maximum plate and screen voltages should be 250 VDC. A physical examination of the tube shows that it should handle more than that since the 46 has a maximum rating of 400 VDC. I called a friend of mine who is a retired tube designer at Eimac and he knows the 47 well from his early career (1940's). He said that it should be OK at the ~280 VDC that I am running here. The 47 is very similar to the 46 but, as I said in my earlier post, but it has a different grid structure and is a pentode instead of a tetrode. The filament and plate seem identical. The 47 has a bigger plate assembly then the 45 and the 45 is rated at Pd = 10W. I am guessing that the 47 is about 11-12W Pd as this specification is not on any data sheets. Finally, the current of 39.5 MADC is divided between the plate and the screen which are tied together for low mu triode operation (I'm guessing that the Mu ~ 6). In this case screen current is about 6 MADC which makes the plate current 33.5 MADC. This results in the screen dissipation being at 1.7W and the plate dissipation at 9.5W. These should well be within the tube design centers.
The 47 is also a great looking tube and as soon as I can find four new globe 47's I'll put them in. One other neat little point about the globe 47. The gettering that was used in this tube can cause it to flouresce slightly under operation as stray electrons strike the glass. This does not mean that the tube is gassy (a gassy tube would have a blue glow within the tube elements). It looks very nice if you can get lucky and have four tubes that do it.
The sound, well I'm going through listening tests now, but initial results are certainly better than the 2A3's and very similar to that which could be obtained with 45's."

I've updated the log with the full text and pictures of the 47.

[bcherry]

Does anyone know the European equivalent to the 47?
Brian