YBA-1A mkii clone with 2x KT88

[Burnsy]

Hello everyone, occasional lurker, first-time poster here.

I'm setting out to build a 100W-ish bass guitar amplifier inside the carcass of a Univox 1246. Something along the lines of the Traynor YBA-1A mkii (mostly plexi clone with two power tubes, high B+), I'm thinking 2xKT88 should do the job.

Right now, I'm planning on using a Hammond 1640R (5k ohm primary) for the OT and some PT with around 400Vac to gimme about 560V B+, ss recto, cap input.

My first question is, is the standard long-tailed pair PI going to provide enough juice to drive this puppy to max output power? I've been reading that KT88's in particular require a large input signal voltage so maybe just copying the Traynor PI design won't get me there.

I'm looking for a sound that gets a bit gritty towards max output, but not total mush.

Also, thoughts on PT choices- Do you think a Hammond 278X would provide enough juice with its 200 mA DC current rating? I know Hammond tends to rate things conservatively. The 278CX is not that much more pricey for more than double the current rating, the main cost is the 5.5 extra lbs.

Another option would be this Antek toroidal AN-4TK400, which again would be way more juice than I really need, but lighter than the 278CX and half the price.

Yet another option would be this Antek AN-2T230. I would wire the two primaries in series to get a 630Vish B+, then use the midpoint get a 315-ish voltage I could use for the screen grids and the preamp supply... how much would that lower screen voltage affect the sound and power output?

thanks in advance for your thoughts!

[loudthud]

The YBA-1A actually uses the 278CX. The same transformer as the YBA-3. I think the 278X would work but run quite hot. The OT went through several design changes. The ones I have measured are pri: 3800, sec: 8. The closest Hammond is the 1650N, 4300 ohm primary. A prototype I made using 6550s with the 1650N measured at 80W ultralinear with 600V B+. Driving a single pair of tubes isn't a problem. A little tweeking on the PI will get you there. The LTPI is a little short on gain but otherwise is ok. I ended up using a 6AN8 but not totally happy with the result. I haven't used the 1650R yet. I think the reduced screen voltage will make the output tubes easier to drive.

Because there aren't alot of examples to go by, consider this an advanced level project. Oscilloscope required.

[Burnsy]

Thanks for the pointers, Loudthud. I just thought I'd post back to update that I have finished this baby, except for the faceplate. I ended up using the toroidal Antek PT (AN-4TK400) and the Hammond 1650N OT. I was hoping to come up with something that would beat a mid-90's Ampeg (SLM) V4BH (4 x 6L6) that I have in the bass power department, and I have to admit it comes up a bit short in that regard. The Ampeg still has the bigger, cleaner bottom end. But MAN is it a nice guitar amp.

I stuck pretty close to the Traynor schematic and didn't attempt anything exotic to get a higher B+ (I get 545 to 569 V, depending on the Arizona summertime electrical grid load) and beefed up the screen dropper resistors a bit. I included a few bells and whistles, like switchable V1 cathode bypass caps on each channel, and built in two different tone stacks (Traynor stock values and Marshall "standard" values), selectable via 3PDT switch on front panel, and also a switchable cathode bypass cap on V2B for an extra gain boost.

I put some guitar clips of this baby up on that Youtube contraption if anyone's interested

YouTube - Homebrew Plexi 100W Tube Amp 6550 Judas Priest Sabbath Lizzy random noodling plus a CUTE BABY
YouTube - Homebrew Plexi tube amp KT66 Traynor Marshall plus a CUTE BABY

[zhyla]

Aren't you concerned about your kid's ears? A bit young for hearing loss if you ask me.

[Burnsy]

Of course I am concerned for my son's ears. This is why, as noted in the on-screen text in the second video, the speaker cab and microphone are located in a separate room down the hall with the door closed. The audio on the clip is from this microphone, not the camera. Also, a hotplate attenuator is used, set to -8 dB. Thanks for asking!

[SonOfCain]

Thanks for the pointers, Loudthud. I just thought I'd post back to update that I have finished this baby, except for the faceplate. I ended up using the toroidal Antek PT (AN-4TK400) and the Hammond 1650N OT. I was hoping to come up with something that would beat a mid-90's Ampeg (SLM) V4BH (4 x 6L6) that I have in the bass power department, and I have to admit it comes up a bit short in that regard. The Ampeg still has the bigger, cleaner bottom end. But MAN is it a nice guitar amp.

I stuck pretty close to the Traynor schematic and didn't attempt anything exotic to get a higher B+ (I get 545 to 569 V, depending on the Arizona summertime electrical grid load) and beefed up the screen dropper resistors a bit. I included a few bells and whistles, like switchable V1 cathode bypass caps on each channel, and built in two different tone stacks (Traynor stock values and Marshall "standard" values), selectable via 3PDT switch on front panel, and also a switchable cathode bypass cap on V2B for an extra gain boost.

I put some guitar clips of this baby up on that Youtube contraption if anyone's interested

YouTube - Homebrew Plexi 100W Tube Amp 6550 Judas Priest Sabbath Lizzy random noodling plus a CUTE BABY
YouTube - Homebrew Plexi tube amp KT66 Traynor Marshall plus a CUTE BABY

This amp sounds great.

If i might ask, what was the total amount of $ spent on this project?

Also, What schematic did you use, can you possibly link it?

[Burnsy]

Hey thanks!
Well, the big ticket items were the transformers (~$100 for the output, ~$55 for the toroidal power), then there's the filter can caps (maybe $25 total), and everything else (jacks, switches, tube sockets, capacitors, resistors, tagboard, other odds and ends), say about $75 more. Then of course, the tubes. I've tried out some lightly used Sovtek 6550WE's, some Sovtek KT88's, some winged-C 6550, and some chinese KT66, so I've spent well over $100 on output tubes. I should also point out here, that on further playing and observation, all of these tubes EXCEPT the KT66 have exhibited glowing of the screen grids under heavy signal. Yikes! So I either need to beef up the screen droppers some MORE or just use KT66, which sound great for guitar, but aren't really seeing the right OT primary impedance for optimal power. Anyway, to make a long story short, If what I really wanted was a YBA-1A mkII, and I value my time at any reasonable amount, I probably would have been better off just buying one on ebay!! But of course I wanted the experience and challenge of building another amp (this is my second amp more or less from scratch) and I gotta say I enjoyed it quite a bit. Expect to spend at least $300 before you're done.

I pretty much followed this schematic
http://www.lynx.net/~jc/700912_YBA1_A_Mk2.gif
Good luck!

[Steve Conner]

You might want to try moving your speaker up one tap on the output transformer. See if this stops the glowing screens and gets you some more output power. Ie, if it's an 8 ohm cab, connect it to the 16 ohm tap.

[pdf64]

Steve, I've noticed that effect also, ie as the load impedance goes up, the screens dissipate more power. Any idea about what the mechanism for this is? Peter.

[reverendbk]

I have a YBA-1 running Winged C EL34's and had to upgrade to 1.5K screens. The sound didn't suffer.

That is a great sound there, especially the KT66's. Very impressive, especially for your second build!

[Burnsy]

I ought to try that out- too bad I was too lazy to wire up the 16 ohm connection from the OT! I didn't think I'd use the amp other than with 4 ohm (for bass) or 8 ohm loads. I hadn't noticed any glowing screens with the 4 ohm bass cab connected, mostly because I don't crank the thing nearly as much for bass as I do for guitar. Also, my power attenuator is 8 ohms. I would just dime the thing with the 4 ohm bass cab connected to the 8 ohm output and look at the screens but... I just can't seem to get the wife and baby out of the house whenever I'm home!