Can a Traynor YBA-1A with KT-88’s be increased beyond 50W/8 ohms?

[nevetslab]

Actually, I didn't know about these vintage Traynor sites, so many thanks on providing those...will check them out I continue searching for the hidden looney within, not allowing the higher power output.

From all the tests run on the OT, as well as listening to it on both 8 ohm & 4 ohm speakers, playing bass thru it, I'd find it hard to believe the OT is faulty. I know it's original. Test set ups produce consistent results, with instrumentation in agreement with one-another. When I first started in on the restoration project, I thought the OT looked a little small, though for a 50W amp, not so much. While I haven't run a full power-bandwidth test on the transformer, I did take it down in steps to see where it began 'folding up', which was 30Hz at 50W/8 ohm. Usually, OT's are good for a lot more power once you're a couple octaves or more higher than where the LF saturation cuve begins. I'm more used to seeing that behavior on autoformers, which you can run a considerably more power thru them than they're rated for...as that rating is part of the LF design limit.. Just have to watch the temp rise. Here with the OT inside the NFB loop, you won't be able to do that.

I haven't gotten around to doing temp rise on this, but I don't see a reason to do so thus far. But, the mystery continues. I don't see anything silly or stupid that I've done. Unless it has to do with the Grommes 260A Series Regulator circuit I cobbled together for the Screens. There is a 470 ohm 10W resistor from the plate supply to the 2nd stage filter, which ONLY runs the Screen Regulator. I inserted a scope photo of that supply when the amp was running 1dB over 20V (22.4V/63W 8ohms, 10% burst power. I've re-inserted that image here, along with the images showing 50Hz 1dB over-drive 10% burst power, showing power supply sag. Current schematic for this amp with the mod's in my previous post.

[nevetslab]

[QUOTE=Leo_Gnardo;354654]Had whopping big transformers and a solid 550W without much sag. Delivered @ 85W at clip.

That's in the ballpark for what I'm looking to fetch from the power supply. Now, I suspect this amp IS the smaller OT. From the power curve one could draw from the output power vs load vs plate resistance data I provided in my post of 8/2/14, run at 100Hz signal, the KT-88's with this transformer peaks at around 66W @ 11 ohms, and 5.1k plate resistance. I hope that's not my limiting factor, but could be. Here's the current schematic of this rendition of the amp.

Traynor YBA-1A Mk II Amp Schematic-1.pdf

Do you recall any of the details you did in setting up your early 70's YBA-1A Mk II with KT-88's? Make any changes in the LTPI circuits? leave the 100k NFB resistor or lower it? Your 85W output was, I presume 8 ohm load, steady state.

Yup....I put in a couple decades at BGW...had excellent chief engineers that I learned from...Mike Robbins, Duke Aguiar to name a couple. Company made a couple 'wrong turns' after the disco boom, and went after the commercial market dominated by Altec instead of continuing to push the power envelope when so few amp companies were in the market place. Soon became a 'nitch company', always on credit hold, and after an untimely death of Mr BGW, it's continuation bordered in hope while trying to get more affordable products out there. We did very well in the Cinema market. Company finally went belly up, and was bought up by Morris Kessler of Advanced Technoligies, Inc (of SAE fame, if memory serves). I think that chap you contacted was a brief repair tech BGW had.

[Leo_Gnardo]

Do you recall any of the details you did in setting up your early 70's YBA-1A Mk II with KT-88's? Make any changes in the LTPI circuits? leave the 100k NFB resistor or lower it? Your 85W output was, I presume 8 ohm load, steady state.

I think I left everything pretty much alone except for: Put in a variable bias control and scaled the bias for KT88/6550, fresh hi voltage & bias filter caps as well. Made the mid scoop mod by jumpering 2 leads on the mid pot. The output tubes IIRC were JJ's and showed no stress so I figured good to go & so far it is. Yes it ran 85W to 8 ohms and would do it all day long if asked.

BGW, yeh I noticed their product line went to all install-type amps with screw-block connectors. Plus the competition had caught up like Crown, QSC, even Peavey. And new competitors besides like Carver and Crest. Still see some around, still working. Goes back a little way but I caught one of the newly re-formed Little Feat shows @ 1994. Kenny Gradney had a BGW 500 powering his bass rig. Of course he sounded awesome as always & I got a hello and a handshake after the show.

[TimmyP1955]

The difference between 85W and 55W is just under 2dB. Hardly worth worrying about.

[nevetslab]

True enough, but, after all the effort AND having large PT present, why the dickens not? It's already on the onset of clip at 50W visibly. On the analyzer, it's getting ragged 3dB below that, so it's more of wanting the clean headroom if it can be obtained. Want more? Add an external heater XFMR, taking that load current out of the PT. KT-88's with 600V plate & 300V screens will yield 100W. I knew I couldn't quite get there, but with big bottles and only 50-55W? That is admitting defeat.....of course, that's just me. I'm half tempted to pull the OT, remove the cover to see if Traynor had a 16 ohm tap that they chopped off, it having been found on an earlier YBA-1, same plate impedance as this OT. I'd run a fresh set of curves with the other tap to see if there's a better match at that point.

[loudthud]

I think the 1K screen stoppers are limiting plate current.

[nevetslab]

Good point. Easy enough to bypass them & see. Thanks

[nevetslab]

I haven't yet had a chance to check out all the other Traynor mod sites g-one posted, so more reading to do. I haven't even gotten to the tone circuits. First things first, but I'm not all that pleased with what's there. Lacks character, lots of room for improvement....too thick in the middle for sure.

I'll try 'loudthud's suggestion of removing the 1k screen resistors, as they're probably not needed, running off their own regulator. The Grommes 260A circuit didn't use them. I left them in for the initial trials.

Compact Monitor Systems was working with Little Feat back then, handling their stage monitor systems......all of John DeBord's amps for his screamin' 3-way wedges were my BGW Compact Monitor TRI-AMP Internally tweaked for his moniitors, so panel controls ran wide open, but allowed for quick system check-out. Made for the road, and still out there kickin' A. That amp spawned the GTC, having more headroom than the GTB, but the GTA was the sold workhorse for quite a few years. We were just getting to switched upper-tier'd output stages when it all came to a grinding halt. Too little too late.

[g1]

Is your OT the A1327 like the one shown here? : http://music-electronics-forum.com/t32329/#post355660
That looks fairly hefty to me.

[nevetslab]

Yup.....that's the same one A1327. At 50W 8 ohm output, 30Hz is where the onset of saturation begins....distortion begins to rise rapidly, waveshape begins to take on non-sinusoidal appearance the lower I go in frequency. I haven't yet done any characterization of it at higher power levels & higher frequency, but usually, they can handle a lot more power if you stay out of the LF saturation area (temp rise not withstanding). it's on my list of things to look at, while i still have this OT in use.....see where the saturation curve is at 5k or 5.5k plate resistance (11 & 12 ohm secondary loads).

My 5-string Fender Jazz Bass with the Low B poses an issue when I finally get to the 75-85W power levels with the output stage. Always something, eh?

[g1]

I think I read in this post that you are testing at 100Hz? I would guess all the big power numbers Traynor claimed would have been done at 1khz, or 400hz.
It would be interesting to see what you get at 400hz. It's conceivable that all the gains made by sellers of "upgrade" OT's are in extending frequency response, especially in the low end. Marketing them in this way (with actual specs showing the gains) would give them back some of the credibility they are lacking, and help people justify the big price difference. You probably all know the "upgrade" transformer company I'm referring to.

[nevetslab]

While some of my power measurements have been done at 100Hz, most have been at 50Hz or 40Hz....getting dangerously close to the LF core saturation area. Earlier today, while setting up the Traynor to run some Tone Stack response plots, I loaded the amp at 16 ohms (resistive load), and looked to see if it changed the onset of LF core saturation at all, from that run with 8 ohm load. It was comparable to the 8 ohm measurements. Only way to get the linearity back was to lower the output level. Drop it down enough, and 20Hz is no trouble at all.

I'll be running more power output data soon....and see what the differences are at 400Hz & 1kHz. 400Hz fundamental is actually beyond the fretboard on a 20-fret 4-string bass. And, of course, getting good numbers octaves away from where your core begins begging for better material and larger core size, it's a bit misleading.

That's why I made the earlier comment about the size of this stock A1327 OT looked a little small, though for 50W power, about right. It probably will be my biggest limitation, even after getting 75-85W output from the bigger tubes & 100W-capacity power supply. That Hammond 1650R OT for running KT-88's/6550's is starting to look essential before I'm done.

[gui_tarzan]

I like the information you've been posting Nev, but that amp G-one linked to is mine and the OT is much thicker than any other 50w amp I have. Not to mention that enormous PT. Since it's a 1A MkII circuit, wouldn't it stand to reason that it is capable of handling the extra power with ease?

[nevetslab]

I would like to think so, from the power supply capacity and headroom it demonstrates. I'm finding more and more the weak link may be the OT...the A1327 that we confirmed. Once I resolve the 'hitch' preventing higher output swing into an optimum load ABOVE the LF saturation corner frequency...which may just be removing the screen grid resistors still left in place from it being the conventional EL-34 output stage.....and find I'm finally getting into the 80-90W power level, I'm willing to bet there's not sufficient core size/wire size to get full power at 30Hz with this OT. Low B on a five string bass is 30.9Hz....funny waveshape, but fundamental non-the-less.

I haven't yet contacted Hammond to see if they have engineering data on their 1650R OT optimized for 100W 5k pri Z KT-88's/6550's, with regards to where it goes into core saturation, and perhaps temp rise. There may be data already run on that OT amongst the forum members.

[nevetslab]

I think the 1K screen stoppers are limiting plate current.

I shorted out the 1k/5W Screen resistors, to see if that was a limiting factor in reaching higher output power. It made no difference at all. I got the same 31V RMS output into 16 ohms @ 4% THD...whether I had the resistors shorted out or not. With them in place, I measure 12V across one, and 9.6V across the other (12mA & 9.6mA respectively being sourced from the 300V Screen Supply regulator.

I won't have time to pursue this further today, or probably this week, having other bench work that's paying rent for me. I mostly pulled the amp out of the case to revise the Tone Stack based on input from MarkusBass (different thread). It was a good guess...wish that was the key.