Traynor Bassmaster (YBA-1) fixed (nonadjustable) bias- problematic?

[GerryJ]

Hello guys, I have two old 1968-1971 Traynor YBA-1 bassmasters, the ones with fixed bias, but no bias pot. (The big brother, YBA-IIA Mark II, came stock with an adjustable bias pot). Both amps have been competently restored recently, work perfectly. One of the long-time power tubes (EL-34) finally shorted out, so I've looked at my stash of replacements, and out of curiosity started checking total plate amperage with a Eurotube bias probe. Here's what I saw, listed as low to high in current to the anode/plate:
Tesla (Slovakia) EL34s - 40 mA. GE NOS 6CA7s- 50 mA. Svetlana EL34s- 53 mA. Tesla (Ruby tubes) EL34s- 57 mA. (All the foreign tubes are 'NOS', acquired 20+ years ago).
All the tubes look and sound fine when I put them in- no hint of any redplating whatsoever.
They saw EL34s should be biased for ~ 38 mA. They also say over time as power tubes weaken, their plate current decreases. I have a pair of Teslas that were mates to other tubes for 3-5 yrs, they run 32-40 mA in these amps (i.e., a lot lower) so maybe with new tubes it's normal to see a fairly high initial plate current?
I don't push power tubes hard in these amps, as I try to limit the volume to 95 dB at most, and I don't use an attenuator.
I'm in the "if it ain't broke, don't mess with it" school, and I haven't read about people having problems with these amps, even though bias is not adjustable- which is also curious (at least to me).
So my question is, should I just keep the amps the way they are, or should I really consider getting a bias pot installed to bring down plate current to ~ 38 mA or less? Thanks!

[xtian]

I would certainly install a bias pot, and 1R bias-sense resistors to make things easy. I tend to bias closer to 50% of max dissipation, rather than the 70% target of yesterday, preferring my power tubes to last a long time.

[Stevem]

Make your life easy and your amp better and get a London power bias adjustment kit.

How hot you bias the outputs can certainly eat them up faster depending on how loud you play the amp.

On the other side of the coin a hotter bias can make the amp sound fatter and provide an increased ratio of output stage clipping to preamp clipping .

Let your ears and fingers be your guide, but there’s no need to ever bias higher then 70%.

[GlideOn]

Installing an adjustable 25kl bias pot in these is dead easy as there's plenty of extra eyelets on the board. Just substitute the 56k for a 22k and put the 25kl pot in series with it.

More importantly, is this YBA1 still stock? Did it come with EL34s originally or did it come with 7027s as it shows from the early to late 60s schematics?

If it currently has EL34s, that would mean pins 1 and 8 jumpered both power tube sockets and going from 150k to 220k bias resistor off the diode network. Also, they really need 1k/5w Screen Resistors in place of the 470ohm. 5.6k grid stoppers also help. These will allow EL34s to work within their best ranges.

Let us know if there are any mods to the amps already as it is actually rare to find one dead stock these days.

[GerryJ]

Thanks! The '68 bassmaster is stock except for a master volume mod. I'm the original owner (!) and it came with American made 6CA7s, which lasted over 14 yrs.
The '71 bassmaster is unmodified & likely also had 6CA7s originally.
I'll have to check the schematics & amps.

[GerryJ]

no mods on the circuitry (outside of the master volume added on one of them)- power tube screen grid resistors are stock - in this case ceramic, 470 ohm, 10 watt screen resistors in one amp, 1k 12 watt screen resistors in the other. and yes, EL34s with pins 1 and 8 jumpered.

[GlideOn]

Stock YBA1s are typically very clean sounding amps:


4M volume pots.

100k mixers.

270pf/47k treble slope.

.1uf coupling caps.

470k/470r/470k Phase Invertor with a 6.8k tail.

100k Grid Leaks.

470ohm Screen Grids...10w at that.


These are Silverface Fender values a full decade and a half of Silverface Fender era. Pete and crew really were innovating quite a bit and admirably eeked out a large amount of headroom from a 40w relatively compact bass amp head.

I really don't think being able to tailor bias the power tubes hotter/colder will matter much in this stock circuit as it will sound clean no matter what.

Of course, modified for guitar is where these really shine. Then adjusting up the bias starts to make sense.

Point being: if you're gonna crack it open to add an adjustable bias pot, you may as well change a few other things too to take advantage of the amps' fullest potential. You have a Marshall killer on your hands, sir.

P.S. - These are some of the sturdiest, toughest amps ever made too. They were thrown off of second story factory building to test for durability and all of them fired back up without any issue. If these were ever "problematic," I sure as hell haven't seen it or heard of it.

[GerryJ]

Thanks! Great info!!

[martin manning]

I really don't think being able to tailor bias the power tubes hotter/colder will matter much in this stock circuit as it will sound clean no matter what.

I would add the bias trimmer just to have the ability to accommodate tube variation.

[GlideOn]

I really don't think being able to tailor bias the power tubes hotter/colder will matter much in this stock circuit as it will sound clean no matter what.

I would add the bias trimmer just to have the ability to accommodate tube variation.

Why stop there?

At the very least split the V1 cathode, make it 1.8k/25uf for normal channel, 2.7k/.68uf for bright channel.

If you want to go further, change the 100k mixers to 270k, put 470pf cap over one or both, put a 1uf over V2 Cathode.

You can keep the coupling caps the same, but the above can be done with Carbon resistors, era-correct ceramic or film caps to keep it all looking original. Maybe $20 worth of parts

Now you'll have two usable, distinct channels that you can either foot switch between using the input jacks and A/B/Y pedal or blend both. They'll both be fuller, livelier than before and guaranteed to dominate in any room you play it in.

There are very, very few amps that can stand toe-to-toe with a modified Traynor. They are that good

[LOUDthud]

Stock YBA1s are typically very clean sounding amps:

Have you ever played one ? I bought a new YBA-1A in 1970. It was the earlier version with the rounded cabinet, not the flip top baby buggy bumper version. If you wanted clean sound on the bright channel, you had to set the Volume pot on about 1. I played bass through it setting the Volume on about 2 or 3. It had 1000pF for a bright cap on the bright channel Volume and 1000pF for the treble cap in the tone stack. Very bright and aggressive sounding. With the 4 Meg Volume pots, it had more gain than a 5F6A even considering the 8 Ohm output transformer. At that time there were no Marshall 2203 or 2204 amps, so it was among the highest gain available. Only weakness of the line was the speakers available, Marsland and Norelco.

The YBA-1A and YBA-3 used the same power transformer. Pretty much a stock Hammond 278CX. Because the transformers were hanging upside down, they would bend or break the end bell mounting tabs. Another weakness of YBA-1A and YBA-3 amps was the bias supply. No bias winding so it was the big resistor off the 800VCT secondary. IIRC is was a 470K 2W carbon comp. In standby mode (lifted center tap) these resistors see over 1000V peak and eventually drift up in resistance lowering the bias Voltage.

Remember, these amps have no Master Volume and any amp that did at the time sucked.

[GlideOn]

Stock YBA1s are typically very clean sounding amps:

Have you ever played one ? I bought a new YBA-1A in 1970. It was the earlier version with the rounded cabinet, not the flip top baby buggy bumper version. If you wanted clean sound on the bright channel, you had to set the Volume pot on about 1. I played bass through it setting the Volume on about 2 or 3. It had 1000pF for a bright cap on the bright channel Volume and 1000pF for the treble cap in the tone stack. Very bright and aggressive sounding. With the 4 Meg Volume pots, it had more gain than a 5F6A even considering the 8 Ohm output transformer. At that time there were no Marshall 2203 or 2204 amps, so it was among the highest gain available. Only weakness of the line was the speakers available, Marsland and Norelco.

The YBA-1A and YBA-3 used the same power transformer. Pretty much a stock Hammond 278CX. Because the transformers were hanging upside down, they would bend or break the end bell mounting tabs. Another weakness of YBA-1A and YBA-3 amps was the bias supply. No bias winding so it was the big resistor off the 800VCT secondary. IIRC is was a 470K 2W carbon comp. In standby mode (lifted center tap) these resistors see over 1000V peak and eventually drift up in resistance lowering the bias Voltage.

Remember, these amps have no Master Volume and any amp that did at the time sucked.

Yes, it's my main amp. 1968 model with choke, no bumpers, solid state rectifier with bright red diodes. Came stock with Jan 5881 tubes and Roberts/Blackburn 12AX7s.

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I played with it stock for a out 2-3 years as a pedal platform and sounded really damn good...until someone brought a DSL50 into the room and blew it away.

I also went through a Marshall DSL50 of my own, never could get it to sound good as that other one. Rocker 30, Vox AC50CP2, Gibson Skylark, Gibson Discoverer, Gibson GA-200 (still have, also modded). None of them were as authoritative as the Traynor however. I underwent a revelation when I discovered people were modifying their YBAs to be more like Marshalls. It badly needed caps replaced at this point anyways.

So it sent me down the rabbit hole. A few years later, well...

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Now it's almost an entirely Marshalled circuit. Still has all 3 original transformers with no signs of slowing down. Kept all the original Mustards (they sound best anyways), tried very hard to keep the original spirit of the amp in place by using period-correct components and gavitt/pushback wiring wherever possible.

...basically an ultimate old-school Marshall where you have: JTM45, a SuperBass and a JMP depending on where you set the switches or push/pull knobs. I didn't drill any new holes, just used push pull or stack/concentric knobs, Jazz Bass knob for lower and Chicken head for top - just like the EVH amps do.

Post-PI MV and Pre-PI MV for bedroom, practice or stage.

Also has the JMP Low Input for clean sounds, minus the speaker to gain some volume back. Incredible cleans at high volumes when paired with a Keeley Empress.

Paired in my den room next to my PC with a custom oversized front-loaded 2x12 with Monotone BV-25M and BV30H speakers.

Also have a garage setup too with a 4x12 loaded with WGS ET65/Vet30s. I'll move it on top when the lads come over to jam and really open it up, rarely using pedals these days - save for low input effects - I let natural amp gain do most of the talking.

However I prefer the 2x12 with the Mojotones TBH. They are crisper and snappier without being fatiguing. Heavenly combination.

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[LOUDthud]

That power transformer looks huge like the ones I see in YBA-1A and YBA-3. Hammond catalog says 278cx is 4.56 high, 3.75 wide (the Es and Is) and 5.00 Deep (end bell to end bell). The only YBA-x I've seen with a choke was a YBA-1 with no convenience outlet. I'd have to dig it up and look but I don't remember the filter caps being near the input jacks.

All the YBA-1s I see (possible exception the one with a choke) use a power transformer with a 300V winding to a full wave bridge (B+ around 430V) and a 60V winding for bias.

[GlideOn]

It is huge, but the 1A is bigger believe it or not! Don't think the 1As has chokes IIRC, don't think there would be room for them, lol. Weird amp that is very high voltage, very hard on tubes, very hard and brittle sounding, IMHO. The YBA3 makes more sense using x4 power tubes normal instead of running the piss out of just 2.

Yes, the filter caps were annoyingly right behind the volume pots and I reckoned they would needed to be moved if I were to pump anymore gain out of the circuit + minimal shield wire for the input and LarMar Master Volume. I had to dremel them out to free them, lol. Plenty of room on board once I moved the bias filter caps and new bias pot immediately adjacent to the diodes (6 of them, lol). PI and Preamp on the board "Dynamite stick" cap bracket I kept but now a CE 80uf + 30uf + 20uf for 130uf total @ 525v. Brings the bass!

Mine didn't have a bias voltage tap, but I didn't really need one with the diode setup as stock it taps it already I just added IF5408 diodes instead which produce slightly less hiss and sound better, imho. I just did the 220k bias feed for the EL34s I was planning to use, jumpered 1, 8 pins together, 1k/5w for the Screens and 25k bias pot with 27k resistor tail. Have to open the amp to bias though, in hindsight I would install upside down in the capped off cover/rectifier slot to adjust from outside the head. Might as well be fixed resistor if it sits in the amp!

I'm not even totally sure what ma voltage, but roughly 10-11 o'clock on the pot taper. Sounds about perfect. I tend to stick to the Shuguang EL34Bs these days, ridiculously good tubes for practically beer money. Raytheons in the rest of the preamp, still going after 10 years.

Reliable as the week is long and as quiet as a mouse fart.

[GerryJ]

Thanks guys. I will probably get bias pots installed on the YBA-1 amps, for my own convenience but mostly for whoever may get the amps in the future. Reviewing and re-educating myself on the famous 'bias' topic, I noticed how the 70% rule (use 70% or less of current estimated to be present at maximum plate dissipation, defined as watts/plate voltage) is more of a 'guideline' than a 'rule.'
In my YBA-1A Mk II, which does have an adjustable bias trim pot, my trim pot is almost (but not completely) counterclockwise (i.e., allowing lowest cathode>plate current flow, 'cold') and it shows a plate current of 38 mA, which is 81% of assumed maximum dissipation of 47 mA (25 watts/537 V). Playing this quite loud (over 100 dB) but only with a touch of nice warm harmonic distortion, in a dark room the plates stay completely dark, no hint of any redplating whatsoever.
I realize tube life in general might be longer if I somehow got plate current lower on this pair of Tesla EL34s, but gosh there can be such variation- like I said in my first post about testing NOS Tesla EL34s in my non-adjustable fixed bias amps, the plate current varied from 40 mA to 55 mA even though they were all new Teslas! I've seen posts from knowledgeable people saying if your tubes aren't redplating and it sounds good, you need to accept that the range of 'acceptable bias' is quite broad.
Question- does plate voltage vary from tube to tube also? I'll be checking actual values soon as I'm picking up a Eurotube probe that directly measures plate voltage as well as current, in a hands-off fashion (I'm not ready for 500 volt work yet). So far, I've just used the plate voltage listed in the schematics. As well as the "25 watts" that's listed universally for all EL34s and 6L6s, for that matter! - thanks.