Any ideas on Ultra Linear designs using Beam Power Pentodes?

[ThermionicScott]

U have some reference for this ??? Do you even know how a screen grid draws current ; in any type of tetrode or pentode ? U ever crack open a power tube and see how it is built ?? Just, u know, asking the questions......


-g

Assuming that's a serious question, here's a primer, from one of the designers of the beam power tube: http://www.pmillett.com/tubedata/beam_power_tubes.pdf

- Scott

[CitizenCain]

I've had good results with EH 7591a's in ultralinear using the Triode Electronics "deluxe reverb" 6k6:8/4 output transformer. My amp matches the operating characteristics on the datasheet almost exactly (around 400 volts on plates) and I haven't had any problems even after extended playing time into a hotplate and/or speaker cabinet. I'm using an antek inc toroid which is pretty stiff- no dropping resistor in series or anything. I seem to recall using 470 ohm screen grid stoppers but I'm not 100% certain.

Wow! I was thinking of building my next with that exact setup, 7591s, Triode tranny and all! Any details on the amp design? Cathode bias? Sorry for the small hijack, but I got excited when I saw my "original" design in use

[Steve Conner]

U have some reference for this ???

RTFDS d00d!

[imaradiostar]

Wow! I was thinking of building my next with that exact setup, 7591s, Triode tranny and all! Any details on the amp design? Cathode bias? Sorry for the small hijack, but I got excited when I saw my "original" design in use

Sorry for the wait, still haven't had a chance to double check what I built so here's my best guess.

Dr. Z route 66 style front end with an EF86, vox ac50 style PI with 100k plate resistors, 47k tail and either 1.2 or 1k bias resistor. I used whatever output coupling caps I had around- they might be 33nF but I don't know for sure. I'm pretty sure the power tube grid leak resistors are 220k. The Antek PT is a 300 volt 100 watt model but somehow it had one 6 volt winding and one 12 volt winding instead of two 6 volt windings so I used the 12v winding, cap and diode bridge to add a little to the B+ voltage. I'm pretty sure it's back biased using a 200 ohm resistor...so it's essentially cathode biased. The back biasing was a leftover from a previous build on the same chassis with much higher voltages- I've been meaning to convert to fixed but haven't had a chance. I don't recall what the preamp voltages were but with only 3 nodes it's not tough to guess and figure out what sounds good-I'm betting they're similar to the Dr. Z values.

I'd be curious to hear someone else's opinion of what it sounds like. My cousin (an Engineer) has used it on a bunch of sessions in Nashville but I don't think it's on any major recordings.

I've since built similar amps with cascodes and 6l6's and they sound great too...but not as good as the 7591's in ultralinear mode.

jamie

[CitizenCain]

Wow, thanks for the detailed reply. That sounds like quite a nice design. I wasn't planning anything nearly that sophisticated. More along the lines of a tweed Deluxe mixed with Ampeg Jet type of thing. I'd love to hear your amp, doesn't have to be a major recording

[Redhat]

Sorry for the wait, still haven't had a chance to double check what I built so here's my best guess.

...


I've since built similar amps with cascodes and 6l6's and they sound great too...but not as good as the 7591's in ultralinear mode.

jamie

Good to know that! I was leaning toward a 6L6 design based on the Fender 70W Pro Reverb. That was until today when I A/B tested a '64 model against a '63 Twin. The Twin blew it away but the amps' owner said he couldn't remember the last time he had changed the Pro Reverb's tubes or if he had ever balanced them since he got it new. (this guy is old and has tons of ancient gear -- all Fender - mostly blonde and blackface).

So maybe his Pro Reverb wasn't properly in UL mode ? The +B is set to 500V which seems too high (according to Svetlana datasheets) but hey, I've yet to make millions selling guitar amps so maybe the Fender guys knew best.

I'll check out the 7591s, Thanks!

RH

[imaradiostar]

I thought I'd mention...I don't think ultralinear and traditional negative feedback are a good pair. I'm sure there are exceptions but I'd try just UL without feedback at the phase inverter to see if you like it before adding other feedback.

jamie

[Redhat]

I thought I'd mention...I don't think ultralinear and traditional negative feedback are a good pair. I'm sure there are exceptions but I'd try just UL without feedback at the phase inverter to see if you like it before adding other feedback.

jamie

That's interesting. One thing both the amp owner and I noticed is that the Pro Reverb has a lack of "sparkle" compared to the Twin Reverb. I did the A/B test using each amps' Normal channel #1 with identical tone control settings and the Twin's volume at around 70% of the Pro Reverb's.

One thing I see on the Pro Reverb is a 0.01 uF cap across the 820 ohm NFB resistor whereas the Twin doesn't have it. This would kill "sparkle" by reducing the feedback impedance by 5% (at 1KHz).

I will definitely investigate your idea. Makes sense.

[Steve Conner]

I thought I'd mention...I don't think ultralinear and traditional negative feedback are a good pair. I'm sure there are exceptions but I'd try just UL without feedback at the phase inverter to see if you like it before adding other feedback.

That's true. UL is just another kind of feedback, and it lowers the output impedance of the power amp in the same way as regular feedback.

Great for tube hi-fi amps, but in guitar applications, too much feedback of whatever kind leads to too low an output impedance, which can make the amp sound flat and dull. Guitar speakers like to be driven by a fairly high impedance, it helps to overcome voice coil inductance and make more upper mids and treble, and lets the bottom end flap and resonate a bit more, too.

The presence control interacts with this in a regular feedback loop (when you turn it up, the output impedance gets even higher in the presence band) but UL kind of kills that interaction too.

[imaradiostar]

To me the thing that made my little amp neat is that even when the bass starts to get loose and sloppy on the bottom it never completely falls apart like most lowish wattage AB1 amps do. This is the case in spite of being cathode biased- I'd imagine fixed would make things even better.

jamie

[pdf64]

'I was leaning toward a 6L6 design based on the Fender 70W Pro Reverb'
I would council against that design, without reducing the B+ to 450V tops, as it's the same arrangement as the fender 75 I've had so much trouble with. The B+ in those is a litle above 500V, 520V when the line is a bit high.
Unless you got a source of sylvania STR 387 6L6GCs, or go with KT88s as Steve advises, in which case 500V UL might be tolerated.

[Groover]

Good to know that! I was leaning toward a 6L6 design based on the Fender 70W Pro Reverb. That was until today when I A/B tested a '64 model against a '63 Twin.
RH

The 64 Pro Reverb is a 40 watt amp. The 70 watt UL Pro Reverb didn't come out until the mid-late '70s.

[Redhat]

Ultra linear .... maybe later

After all the I've read (here and elsewhere) and especially after the A/B test last week, I've punted on the UL idea altogether.

It seems like a great idea in theory, but then so does the idea of SS amps.

Instead I'm going with a Fender Twin topology with a bunch of extra goodies that I've been working on over the years.

Thanks for everyone's input. It inspired a lot of research -- always a good thing!

RH