I got some free stuff...

[dwinstonwood]

Hey all,
A friend was clearing out some stuff he didn't need. I got two AO-63 OPT's (the giant one and the smaller one), the PT and OPT from a Kalamazoo Bass 30, a pair of GE 7591A's, 2 GE 6EU7's, 6 Made in Holland (Hammond branded) 6BQ5/EL84's, two RCA 7189's, and a Motorola 5Y3GT.

Not a big haul, but it was free, and I think I can use most of it.

I hooked 120VAC up to the Bass 30 PT and I measured 360.9VAC from each red secondary to CT no load. The filament secondary measured 3.5VAC to CT on both ends and 7.0VAC green to green no load. So, if I'm not missing anything, it's a good working PT. It's big and heavy. 360VAC with a SS diode rectifier is about 510VDC unloaded. It obviously has the current for two 7591A's, but I don't know if it can feed a pair of 6L6GC's.

I also don't know if the 7591A's are still good; I have no way to test them outside of an amp. But they are rated for 500V on the plates, so the PT under load should be just right for them.

Anyway, what's the general opinion on those Holland/Hammond branded 6BQ5's? Were they considered good tubes? I guess I can try them out in my AC15.

Thanks!
David

[dwinstonwood]

Here's my question: I don't have a low voltage PT, so I was thinking if I safely tape off/insulate the HT secondaries on the Bass 30 PT, I could connect the heater windings to the secondaries of the OPT's and follow the video. What should I do with the heater CT? Or, could I use one heater lead and the CT instead, leaving the other end unconnected?

How would you do it?

Never mind this question. I figured it out. 

Thanks again, David

[SoundmasterG]

I also don't know if the 7591A's are still good; I have no way to test them outside of an amp. But they are rated for 500V on the plates, so the PT under load should be just right for them.

Thanks!
David

7591A's/7868's are my favorite power tubes. They have a lot of gain for a power tube so it is very easy to build a good strong preamp in front of them, and they are easy to get touch sensitivity and lots of harmonics out of too, just like the 6BQ5/EL84, but they have a more even frequency response and more low end capability than the smaller 6BQ5/EL84. If I had them here I would just test them on my mutual conductance tube tester, but you are likely not in the Portland, Oregon area where I live so I guess the best choice for you would be either selling them, or building an amp to use them.

Greg

[bmccowan]

I also don't know if the 7591A's are still good; I have no way to test them outside of an amp. But they are rated for 500V on the plates, so the PT under load should be just right for them.

Do you have a 6L6 amp? You could easily rewire the power tube sockets for the 7591s to test if they are OK.

Anyway, what's the general opinion on those Holland/Hammond branded 6BQ5's? Were they considered good tubes? I guess I can try them out in my AC15.

I've used quite a few of those. They are better than new production EL84/6BQ5 IMO. Likely Amperex.

[dwinstonwood]

Thanks SoundmasterG and bmccowan.

I'm very interested in putting a preamp in front of these 7591A's.

I've been gathering all the parts to build this All-Octal 6V6 6G6-C Bassman (see schematic below). But, maybe I'll just buy a Hammond 194B choke and use the Kalamazoo Bass 30 PT, OPT, and the 7591A's in this project. Looking at the 30 Bass schematic it looks like two 8 Ohm 10's in series for a 16 Ohm load.

[dwinstonwood]

OK, I used Uncle Doug's method to measure the Bass 30 OPT.

I fed 3.5VAC into the secondary and measured 71VAC across the blue and brown primaries. The 7591A datasheet calls for a 6,600 Ohm plate load. Doing the math based on a 6,600 Ohms plate load I got a result of 16.04 Ohms secondary load. The schematic shows a 16 Ohm speaker load using two 8 Ohm speakers in series. This OPT is perfectly matched for the schematic.

Using a single 8 Ohm speaker would result in a 3,300 Ohm plate load which should work with 6L6GC's I think.

I then did the same test on the smaller Hammond A0-63 OPT. 3.5VAC into secondary and 109VAC across blue and brown leads. Doing the math for an 8,000 Ohm plate load gives a result of 8.24 Ohms secondary load. So, this OPT is good for 6V6GT's or EL84's with one 8 Ohm speaker.

[dwinstonwood]

So, I've never bonded with the AC15 that I built a while back. I've tried four different brands of current production EL84's and a pair of NOS Reflektor 6n14n (6p14p)'s that I bought from KCA. Regardless of the tone settings on the amp and my guitar, the treble was always raspy with no harmonic chime. I was also never able to dial in a balanced midrange. I even went as far as rewiring the EF86 socket for a NOS 5879 pentode. That helped a bit. But, I never played the amp because I couldn't get a full frequency range with a clear high end.

Tonight I installed two of the Made in Holland (Hammond branded) 6BQ5/EL84's that I mentioned above. Well, in my case, different output tubes really did transform the amp. I dimed it through my Weber Mini-Mass and now I finally like my AC15.

[bmccowan]

Dw, I've had similar experiences with EL84 guitar amps. Marshall 18 TMB being the worst to my ears. Those old Hammond/Amperex tubes (only played by a little old lady on Sundays) help, as do some old Tungsram tubes from Hungary that I lucked into.
If/when you get bored, another effort worth trying is to convert the power amp for 6973 tubes. NOS 6973s are $$$ but RCA 6CZ5s are the same tube with a slightly different pinout. The sound is richer to my ears than EL84s. But I love EL84s in HiFi amps.

[shooter]

But I love EL84s in HiFi amps.

agreed, I've found the 84's really hate being pushed into hard clipping but keep them in Class A and they will sing sweetly.
 

[elams1894]

I agree with all that has been said about EL84's. In my own experience, in PP guitar amps with a moderately high gain preamp, 3-4 triodes, I have a lot of trouble keeping the 84's stable. Anything past noon on the MV dial, with full gain, the tubes scream into instability. With exact same circuit, 6V6GT's have no problems at all.

I have tried everything, snubber caps, dull caps, smoothing caps, enhancement caps, tube rolling, electrolytic cap replacement, bias replacement, separated preamp from poweramp (different chassis), changed location of OT, local feedback, global feedback. The only thing that brought the tubes under control was replacing every 12AX7 with a 12AU7. And the only 12AU7's that sounded any good with the EL84's was NOS vintage Miniwatts (absolutely wonderful!!). The only EL84's that sounded good were also the NOS vintage Miniwatts, but still unstable, as were Telefunkens, Mullards, Trigons.

I only went to so much trouble as the schematic sounds fantastic in the OD channel up until it becomes unstable. However, any kind of feedback, whether local or global, completely destroyed the high end, which is what the tube is renowned for. In addition, I found that the high frequencies on the clean channel were significantly inferior, in terms of both clarity and chime, when compared the same circuit in a 6V6GT amp. That one still puzzles the heck of me. I cant for the life of me figure it out.

I'm running my EL84 plates at about 310V, so not cooking them like in the Marshall. I've tried the preamp with 200R/25W sag resistor and without. The sag resistor gave me a slightly more pleasant OD with more separated notes and a tad more clarity, for what its worth.

I can only conclude, from my own uber frustrating agonising experience, that the EL84 in PP cannot handle high gain. That the EL84 is incredibly sensitive to higher gain input. As @DW has also observed, one needs to get everything bang on to get an EL84 amp sounding as it should. It is with immense sadness that I have abandoned my aspirations of a killer EL84 PP amp. My 6V6GT sounds infinitely superior in all facets of both OD and cleans. Its my fault entirely, I am sure of it, and I hold no grudges against the tube, its me...not you.

In SE designs, I have had no such problem with exact same high gain schematic, and the tube sounds great. However for as yet unknown reasons, in PP, its a monster fail.

[dwinstonwood]

Thanks for all of the interest friends. I discovered that I had wired a voltage divider into my AC15 for some reason, maybe a concern for oscillation? I don't remember why I added the two resistors (they're in the schematic I posted above). I removed them.

For fun, I also stuck a "Realistic made in Korea" 12AX7A in the PI socket. With the volume all the way up I don't need a Fuzz pedal with this amp. It's kind of cool.

Unrelated, here's a teaser photo: 6G6-C with 6V6's. Oh, and it's all NOS Octal.