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Finally found time to begin the restoration of a 1970’s vintage Traynor YBA-1A Bassmaster Mark II Amp. Bought it new in late ’70. (datecodes inside on the pots: 7023, 7035, 7043…year & week of the year). Last used probably in ’73, then it’s been in storage until I brought it into my shop a couple years ago to age a bit more until now. So, hasn’t been powered up for 40 yrs..
As many of you know, this version is fashioned after the ’59 Fender Bassman with very little variation, apart from a huge P/T, larger O/T, 6CA7 power tubes, higher plate supply (560VDC), cooling fan, only one tone control stack with the Fender Presence Control & Midrange control labeled differently. Much heavier, of course (54lbs).
When I got it onto the bench, my mind said ‘KT-88 tubes’. Though I hadn’t done any checking to see what the single-secondary OT’s primary impedance is. 8 ohm only on the secondary. After isolating the primary windings from the circuit, I found open circuit turns ratio of 21.5:1, and primary impedance (with 8 ohm secondary load attached) ranging from 3.54k @ 50Hz thru 4.19k @ 5kHz, or about 3.8k +/- 10% 30Hz-5kHz.
Before even thinking about powering this amp up (in small steps with Variac over time), I ripped out the 2-wire cord, removed the ‘death cap’, installed a 3-wire power cable & gnd’d that, so the chassis is safe to begin with. It was wired such that you DO NOT install 7027 tubes (pins 8 & 1 tied on the tube sockets). Fixed bias at present. That will, of course change.
I’ll re-cap it, of course. FT dual 100-100/500V’s, 100/450 axials, etc. Sprague 715P polypropylenes in the stage couplings & tone stack, fresh ‘lytics for the bias & cathode bypass stages.
After a lot of time searching thru the forum posts on this website on KT-88 output stages, Screen Grid power supplies, matching tubes to OT’s and lots of threads that spun off the forum posts from brilliant seasoned EE’s, builders, techs and all, I knew I had a chance at getting this one right. I’ll for sure be building it with a separate screen supply, about 1/2 that of the plate voltage, though what form factor I’ll use is still undecided. I’ve been very intrigued by Jon Wilder’s approach with two PT’s, one for the HT & heaters, the other for the Screens & Bias, but also intrigued by a series regulator sourced from the HT supply. Even that ’58 vintage Grommes 260A amp circuit using a 6L6GC as a series regulator to run the screens is intriguing (link from member ‘iamiradiostar’).
A couple MosFET regulator circuits from Steve Conner’s posts are intriguing as well. One from his lower powered SE Corvette Ninja amp to run the screen is good food for thought, and another circuit that evolved from discussion between members ‘R.G,’, ‘Wilder Amplification’, ‘loudthud’, ‘Merlinb’ & ‘iamaradiostar’, all beginning around post # 85 of the Official Dual Rail Thread from Sept 2010. There are great ideas within that long thread, and elsewhere by lots of the contributors.
I haven’t gotten to that point yet…I still have to go thru the load line exercise so I understand it all. And, see where the Traynor’s OT fits into the equation.
I’ve also read the comments from many regarding backing away a bit from the full precision of where it can go to remembering this is still just a Bass Amp, driving loudspeakers that only hit their ‘nominal impedance’ out in the 150Hz octave range (the upper range of the G string on your 4-string bass), and above and below that, the impedance on the secondary can go nearly an order of magnitude higher. And comments regarding when you’re going to deviate from the correct load impedance, go lower rather than higher, as higher impedance can kill screen resistors. The general consensus is that tube amps can tolerate 1/2 the rated load better than twice the load.
I have a number of changes to make on this Traynor YBA-1A. Beginning with their grounding. I’ve never been a fan of dumping the power supply charging current into the common ground shared with the input signal’s ground, succeeding gain stages’ grounds from that point, and the output ground current from the secondary, scattered around the chassis, including soldering bottom leads of pots to their cases and relying on the conductivity of the steel chassis to get them grounded….RF style grounding. I also don’t like pulling load current from rectifiers instead of from the capacitor bank that’s at that node in the circuit. So, the simple grounding gets revised to what approaches single-point grounding, with the chassis ground occurring at the input jack. And power distribution will be from the filters. .
I’ll no doubt deviate from the Fender ’59 bassman tone stack. Also looking to make use of the second input stage’s tube, as I’m wanting to add an insert send/return jack for my compressor/limiter for my bass rig.
The KT-88’s increase the heater current by 1.4A over the existing circuit, and a 6L6GC series regulator (if I went that way for the screen supply) would bump that up to 2.3A. No idea how the PT will respond to the higher demand.
Does anyone out there know what the stock PT on this YBA-1A’s heater supply is rated for? Sometimes, nearly doubling the heater load can raise the coupled mains freq field into the amp circuits quite a bit. It would increase the temp rise of the heater winding, something that I’ll look into just to satisfy my own curiosity, and report those findings.
As many of you know, this version is fashioned after the ’59 Fender Bassman with very little variation, apart from a huge P/T, larger O/T, 6CA7 power tubes, higher plate supply (560VDC), cooling fan, only one tone control stack with the Fender Presence Control & Midrange control labeled differently. Much heavier, of course (54lbs).
When I got it onto the bench, my mind said ‘KT-88 tubes’. Though I hadn’t done any checking to see what the single-secondary OT’s primary impedance is. 8 ohm only on the secondary. After isolating the primary windings from the circuit, I found open circuit turns ratio of 21.5:1, and primary impedance (with 8 ohm secondary load attached) ranging from 3.54k @ 50Hz thru 4.19k @ 5kHz, or about 3.8k +/- 10% 30Hz-5kHz.
Before even thinking about powering this amp up (in small steps with Variac over time), I ripped out the 2-wire cord, removed the ‘death cap’, installed a 3-wire power cable & gnd’d that, so the chassis is safe to begin with. It was wired such that you DO NOT install 7027 tubes (pins 8 & 1 tied on the tube sockets). Fixed bias at present. That will, of course change.
I’ll re-cap it, of course. FT dual 100-100/500V’s, 100/450 axials, etc. Sprague 715P polypropylenes in the stage couplings & tone stack, fresh ‘lytics for the bias & cathode bypass stages.
After a lot of time searching thru the forum posts on this website on KT-88 output stages, Screen Grid power supplies, matching tubes to OT’s and lots of threads that spun off the forum posts from brilliant seasoned EE’s, builders, techs and all, I knew I had a chance at getting this one right. I’ll for sure be building it with a separate screen supply, about 1/2 that of the plate voltage, though what form factor I’ll use is still undecided. I’ve been very intrigued by Jon Wilder’s approach with two PT’s, one for the HT & heaters, the other for the Screens & Bias, but also intrigued by a series regulator sourced from the HT supply. Even that ’58 vintage Grommes 260A amp circuit using a 6L6GC as a series regulator to run the screens is intriguing (link from member ‘iamiradiostar’).
A couple MosFET regulator circuits from Steve Conner’s posts are intriguing as well. One from his lower powered SE Corvette Ninja amp to run the screen is good food for thought, and another circuit that evolved from discussion between members ‘R.G,’, ‘Wilder Amplification’, ‘loudthud’, ‘Merlinb’ & ‘iamaradiostar’, all beginning around post # 85 of the Official Dual Rail Thread from Sept 2010. There are great ideas within that long thread, and elsewhere by lots of the contributors.
I haven’t gotten to that point yet…I still have to go thru the load line exercise so I understand it all. And, see where the Traynor’s OT fits into the equation.
I’ve also read the comments from many regarding backing away a bit from the full precision of where it can go to remembering this is still just a Bass Amp, driving loudspeakers that only hit their ‘nominal impedance’ out in the 150Hz octave range (the upper range of the G string on your 4-string bass), and above and below that, the impedance on the secondary can go nearly an order of magnitude higher. And comments regarding when you’re going to deviate from the correct load impedance, go lower rather than higher, as higher impedance can kill screen resistors. The general consensus is that tube amps can tolerate 1/2 the rated load better than twice the load.
I have a number of changes to make on this Traynor YBA-1A. Beginning with their grounding. I’ve never been a fan of dumping the power supply charging current into the common ground shared with the input signal’s ground, succeeding gain stages’ grounds from that point, and the output ground current from the secondary, scattered around the chassis, including soldering bottom leads of pots to their cases and relying on the conductivity of the steel chassis to get them grounded….RF style grounding. I also don’t like pulling load current from rectifiers instead of from the capacitor bank that’s at that node in the circuit. So, the simple grounding gets revised to what approaches single-point grounding, with the chassis ground occurring at the input jack. And power distribution will be from the filters. .
I’ll no doubt deviate from the Fender ’59 bassman tone stack. Also looking to make use of the second input stage’s tube, as I’m wanting to add an insert send/return jack for my compressor/limiter for my bass rig.
The KT-88’s increase the heater current by 1.4A over the existing circuit, and a 6L6GC series regulator (if I went that way for the screen supply) would bump that up to 2.3A. No idea how the PT will respond to the higher demand.
Does anyone out there know what the stock PT on this YBA-1A’s heater supply is rated for? Sometimes, nearly doubling the heater load can raise the coupled mains freq field into the amp circuits quite a bit. It would increase the temp rise of the heater winding, something that I’ll look into just to satisfy my own curiosity, and report those findings.
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