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

[nevetslab]

After restoring my 44 yr old Traynor YBA-1A, full recap, revised grounding & power distribution, I first set it up with EL-34’s to get familiar with it after all these years. I added 1 ohm cathode resistors on the power tubes, grid stoppers & 1k 5W screen resistors, along with a bias pot, as it was fixed bias before.

I had measured the OT as having a turns ratio of 21.5:1, and a plate impedance of 3.8k +/- 10% from 40Hz-5kHz (3.5k @ 50Hz). Plate supply @ idle measured 570V, and 540 to 520V @ 50W/8 ohm load (20V p-p ripple). The output clips @ 20V RMS (50W/8 ohm) steady state.

I then added a 300V Screen Supply based on an old Grommes 260A Hi Fi Amp that used KT-88’s and a 6L6GB wired as a series regulator from the 2nd stage filter yielding 304VDC at the 1k grid resistors of the 6550 tubes that I used for this second iteration. Revised the bias circuit slightly to give adequate adjustment range, and started with 30mA plate current.

After finding (to no real surprise) I still only had 50W output steady state, I set up my burst sine generator to see what I get at 10% duty cycle, using 50Hz sine for 100mS, -30dB or -20dB signal for 900mS, with a 1 Sec rep rate. I was interested to see if there was greater power output during short burst, and to see what the power supply sag was under full power drive.

I could barely get 1dB over 20V RMS before the output was clipping. Looking at the PI stage and the plate circuits of the power tubes, I found both clipping at the same time. NFB around the output stage is 22.3:1 (100k FB, 4.7k shunt). Measuring across the 4.7k grid stopper resistors, as soon as the amp begins to clip, there’s voltage drop across them.

With a turns ratio of 21.5:1 on the OT, 20V RMS on the output would yield 430V RMS on the primary, plate-plate. Each tube would be supplying half that, or 215V RMS/304V peak. Looking at the voltage swing for each plate on the scope, I see a bit more than that (around 340V pk), though that may just be a reading error on the scope. Still, that’s well below the plate supply of say 520V at the low side of the 20V ripple seen under full power drive.

With the regulation of the power supply, and what looks like a PT made for 100W amplifier operation, I surely have sufficient supply reserves to go well beyond this 50W limitation I’m facing.

Is it the OT’s primary impedance that is too low for 6550/KT88’s? Is it the NFB circuit restricting the max drive? Is it the LTPI circuit configuration within the loop?

[g1]

Have you tried it into a "mismatched" load impedance (16 and 4 ohm)? That would tell you pretty quick if it is an OT primary impedance issue.

[nevetslab]

Only when I had it set up with EL-34's, and only with my shop Ampeg BXE-115HL4 test speaker.....4 ohm cabinet. I haven't yet made any measurements with my load bank at 4 or 16 ohm. I'll have a look when I get back to the shop tomorrow.

The questions are more generic in nature, really. In reading thru LTPI threads, as well as other driver circuits, one comment that caught my attention was from a post by member 'redelephant' in the thread labeled 'Paraphase to fixed bias?', dated 10-4-2010, post #12:

“For maximum headroom at the amp's output, more PI-gain is often better than the opposite. The reason is u want the output to crunch much before the stages ahead. That means the preamp and PI stages are clean even when the output reaches it's max. That means the PI must be able to swing much more voltage than is necessary for getting the output tubes to reach total clip. If the PI does not have gain, u risk getting PI distorting around the same amplitude as the output, and the clean headroom will be less.
I know...Any PI no matter how much it can swing will clip at the same time as the output tubes because of the grids of the output tubes suddenly pull current, and the load becomes a very low impedance. If using a buffer between the PI and output tubes the PI will still swing linearly while the output tubes clip.”

When I measure across the input grid resistors of the power tubes as the amp goes into clip, current flow thru them appears as described in his statement. I don't know about adding buffer stages between the LTPI and power tubes within the NFB loop, though. But, altering what behaves like a peak voltage limiter is what I think I'm after. I'm looking to get more output level that's available from the power supply. I don't have enough to supply voltage to yield 100W, but probably 75 to 80W. Even though that's only about 2dB over what I have presently. That's the goal I'm seeking.

[g1]

Sorry if I'm misunderstanding your last post, but if you pull the power tubes, you will see what the PI is capable of without the action of the power tube grids going into conduction.

[nevetslab]

I have done that just to see there's plenty of voltage swing in the LTPI stage. Once they're back in place, both clip at nearly the same time, I assume due to NFB.

[g1]

I'm hoping playing with the load impedance will make a difference. Once I plugged a pair of 6550's into an Ampeg V4 and got close to 90W by playing with the impedance setting. That was with around 550V B+ so I think you should at least get a substantial increase above 50W. Mind you, that was with NOS 6550's, I'm not sure what the state of modern KT88/6550 is like power wise.
Other than that, I'm wondering if a 12AU would get you more drive.

[nevetslab]

I did try it with a 12AT7 driver. I was still working with an 8 ohm load, but did, of course, find the higher current draw in the LTPI circuit dropped the supply voltage. It was still clipping at the same output level. I didn't have a 12AU7 to plug in to try, but will give that a whirl. The 12AU7 has the lowest amplification factor of the three, but see that's a commonly used driver tube in Ampeg amps. Thanks for the input.

[nevetslab]

Back on the bench with the amp. Checking with 4 ohm load, both steady state and 10% burst power 50Hz & 200Hz signals, onset of clipping is 11VRMS, which is only 30.3W/4 ohms. Checked it next with 16 ohms, where onset of clipping is 30V RMS, which is 56W/16 ohms. Checked next at 12 ohms, where onset of clip was 28VRMS, yielding the best power match of 65.3W. So, I'd say the existing output transformer for KT-88's/6550's use isn't ideal for 8 ohms. Certainly not for 4 ohms.

[nevetslab]

Power Output vs OT Primary Plate Impedance / Load Impedance

After getting the Traynor YBA-1A set up with its’ 300V Screen Grid supply to run KT-88’s or 6550’s, I selected a pair of 6550’s from my available tubes, mostly pulls from servicing Ampeg SVT’s. Found a pair that matched within a mA @ 30mA bias current @ idle.

The Primary Plate to Plate impedance was computed based on first obtaining the turns ratio of 21.5:1 for the Primary to Secondary voltages, unloaded. That translates to 3698 Ohms for the Primary P-P Impedance. I also checked the OT from 40Hz-10kHz for variation of plate impedance, driving the Primary from a power amp (for near-0 ohm source Z) and used 0.5000V as my ref. output level on the Secondary, loaded by a precision 8 ohm 1% power resistor. Then, dialing in series resistance from the 0 ohm sig source to the Primary, stopping when the output at the Secondary dropped to 0.2500V (1/2). That added series resistance equals the primary plate resistance.

It yielded a Primary Plate Impedance of 3.8k +/- 10% from 40Hz to 5kHz.

Then, setting up my huge 4KW Load Bank, having 8 x 4 ohm/500W loads, I ran measurements of Max Output Power @ 100Hz @ visible clip (3-5% THD on the distortion analyzer), and came up with this chart showing where the output power peaks using the stock Traynor YBA-1A Output Transformer, with 6550 power tubes installed.

Load V Out Pwr Out Plate Z Plate Current

4 ohms 11V 30W @ 4 Ohms 1.85k 127mA
8 ohms 21V 55W @ 8 Ohms 3.7k 127mA
10 ohms 25V 62.5W @ 10 Ohms 4.62k 123mA
11 ohms 27V 66W @ 11 Ohms 5.1k 123mA
12 ohms 28V 65W @ 12 Ohms 5.5k 118mA
16 ohms 31V 60W @ 16 ohms 7.4k 99mA

Quiescent Bias was still set at 30mA for each tube. Under load, I selected the higher of the two readings (DCV reading across the 1 ohm cathode resistors).

The Power Output peaks for an 11 ohm load, computed to 5.1k Plate Z (turns ration squared x secondary load imped), which for this was ((21.5) ^2)(11 ohms)

I can’t get very close with the Traynor OT, short of having a cabinet loaded with (3) 4 ohm speakers in series.

Changing to a Hammond 1650R Output Transformer, having a Primary Z of 5k, and 4/8/16 ohm taps would probably be the best solution. Still, at the 30mA bias setting, I’m a good ways off from what I’d expect to get from the stock power transformer. It’s the same PT as used in their YBA3-3 100W amp.

At present, I don’t have the spare cash to spring for it, but this does seem to point to the best load matching with a pair of 6550’s or KT-88’s. When I was working with the bias level at 8 ohm load, I didn’t see any significant improvement while watching the distortion & output signal traces, so I left it set for the nominal 30mA quiescent bias.

I’ve also attached a full schematic of the revised Traynor YBA-1A Mk II as I have it currently configured. I don’t yet have voltage values added.

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

[loudthud]

I think your amp is a 1970. I modified a '69 for 6550s and never got much more than 50W. I attributed the low power to the crummy tubes that were available to me at the time. The '69 has a bigger OT (same primary Z) and there is a 16 ohm tap wire that is cut off where it exits the end bell. I disassembled the OT and attached a wire. From time to time AES has a 10% off sale or free shipping, a good time to get a 1650R.

[nevetslab]

It is indeed a 1970, based on datecodes found on the front panel pots. The capcan's and the long 80uF Mallory electrolytics used a different format datecode that I don't know (used to week & yr--4-digit code). That's interesting about the '69 amp having a bigger OT, AND a hacked-off 16 ohm tap inside the end bell.

Currently AES has it on sale for 10% off, but I'm totally strapped for cash, having spent funds I really didn't have just getting this amp out from under my desk and finally doing one for me instead of clients. I just don't pay myself squat, but it's been very satisfying to see it come together steadily.

I'm still lacking clear understanding as to what is causing the amp to clip. Getting only 50W with a plate supply starting around 560V, while seeing other 50W amps running close to 100V less just bothers me. Seems like I should be able to get more than I'm seeing. Having found where the power output peaks with this Trayor OT & 6550's, next time it's back on the bench, I'll work at that 11 ohm load, and try a fresh pair of KT-88's or 6550's. The tubes I installed here were, I think, not yielding as much plate current in SVT's as the other tubes installed, so.....I was looking for suitable match from my collection of 'pulls' in returning the SVT's back to rental duty (after additional service). Get's really costly pulling the remaining 5 tubes from an SVT when it coughs up a bad one, but over time, there's plenty to select thru.

[g1]

Traynor serial numbers will tell you when the unit was built (at least they did in the 20th century). You need to know the decade which you can figure out from the model. The first digit of serial is last digit of the year. So if yours is from 1970, the first digit of serial will be zero. The next 2 digits of serial is the month. So a serial starting with 008 for a seventies model would be built Aug.1970.
Warranty was easily figured out from serial, with a 6 month grace period to allow for time between build date and sale. Beyond the grace period, a purchase receipt was required.

I'm still lacking clear understanding as to what is causing the amp to clip. Getting only 50W with a plate supply starting around 560V, while seeing other 50W amps running close to 100V less just bothers me. Seems like I should be able to get more than I'm seeing.

It's bothering me too . From your description of the sag, it doesn't sound like the power supply is the limiting factor. But you did get up to 65W by playing with the impedance so perhaps that is the limit of the supply.

[nevetslab]

Thanks for the breakdown of their serial number date code. We did something similar at BGW Systems. I don't have the S/N handy, but do recall first digit was a zero.

Knowing the power transformer on this amp is (reportedly) the same as that used in the 100W YBA-3 amp, it doesn't sound like the PT has run out of poop. I suppose it's time to open the NFB loop and see what's up. In a different thread I started on the LTPI's input cap, a comment about Traynor using 100k as the FB value seemed to be in error. 51k would have been the next logical step up in value, going back to '59 Fender Bassman circuit, where that amp used 27k due to the 2 ohm load. I'll look at that while I'm at it.

Thinking back to when I had the instrumentation set for 20V out/8 ohm load, I had swapped the driver tube (12AX7) out for a 12AT7. The output level dropped to about 15V (from 20V) for starters. And, with the lower Amplification Factor and lower plate resistance of the 12AT7, all the quiescent voltages around the LTPI circuit changed accordingly (have those values written down in my notes). I'd expect similar to happen substituting in a 12AU7, though if revising the circuit values for either tube will push the power output level up, it would be worth the effort. I'd have expected this exercise has been done by others many times, and was hoping to hear what the solution is.

[g1]

What is really baffling is that the 1A was supposed to be much more powerful than the 1 (they claimed as much as double the power ). The 1 had around 450V and was good for 40 to 50W. The 1A had over 100V more on the plates and bigger iron (although OT maybe smaller in later years as noted by Loudthud ?).
Any chance the OT is faulty or non-stock? Or something has gone wrong with your test rig?

You probably know about these vintage Traynor sites but I will post them here anyway:
viva analog Welcome to "viva Analog"
velvet black Velvet Black - Traynor Tube Amps - History, Models, Dating, Links, Information, Amp Maintenance, Opinions, Pictures
traynor tweaks Traynor Tweaks - Modding your Traynor Tube Amp

[Leo_Gnardo]

What is really baffling is that the 1A was supposed to be much more powerful than the 1 (they claimed as much as double the power ). The 1 had around 450V and was good for 40 to 50W. The 1A had over 100V more on the plates and bigger iron (although OT maybe smaller in later years as noted by Loudthud ?).

I'm not good at keeping track of which was which but I did set up an early 70's YBA-1A maybe MkII with KT88's. Had whopping big transformers and a solid 550W without much sag. Delivered @ 85W at clip. Its current owner uses it as part of his quadraphonic guitar rig, and no trouble over the course of at least 3 years.

The "smaller" YBA-1's are terrific for guitar, much like early 70's 50W Marshalls. I set a late 60's one up for 6L6's, @ 45 watts and its owner's getting a good long run out of it. I watched the band he's in as opening act for Dicky Betts a couple years ago and his sound was spot on, just wonderful. Gave DB and his band a run for the money.

If yours is the "small transformer" type then 50W is probably the limit.

BGW eh? Good outfit! You guys made mighty tough amps, and if I'm not mistook for Altec and EV too. What happened? About 25 years ago I called for a replacement power transformer & was directed to order from a fellow not at BGW with a strange name: Phaly Oung in Hawthorne CA. Despite a near insurmountable language barrier I got the transformer ordered and the amp successfully repaired but I'm wondering WTF happened to good old BGW.