Traynor Yba1a Mk II

[chris61]

good day
amp experts,
I always get something fixed when I come here.

I have a question about a varistor on my traynor MKII head
on the schematic it says
note 7
adjust R7A to allow 4 VDC across r25 (470 ohm 10 watt) screen resistor
on the schematic right above (on the screen side) this resistor the circuit is marked +525V
so do I measure 4v over or 4v under?
or what?
anyone cane make sense of what I am asking?

thanks
cp

[chris61]

here is the schematic from the chassis cover.

and a link to maybe get a better view.
http://i872.photobucket.com/albums/a...pse96c723e.jpg

[Jazz P Bass]

So there setting the output tube bias by measuring the screen voltage drop across R25?
Huh.
Who woulds thunkdid.
You could place your meter leads directly across R 25 to measure the stated 4 volts.
Kind of wierd that the schematic states 535V before & 525V after R25.

[chris61]

thanks for the reply
so it is a voltage drop from the 535? to say 531?

[g1]

You are correct, if that voltage is actually 535V. Place one meter probe at each end of the resistor, adjust till your meter reads 4 volts.

[chris61]

great,
thanks guys!
I need to recap this head then I want it to be set up right.

[Leo_Gnardo]

great,
thanks guys!
I need to recap this head then I want it to be set up right.

Good amp! But there are better ways of checking your output tube bias. First & foremost the old trick of putting a 1 ohm precision (1%) resistor in the cathode circuit of each output tube. Each millivolt you measure = 1 milliamp of bias current. You can measure each output tube separately to make sure they're more or less matched for emission. And don't have to stick your meter probes into high voltage, which reduces hazard of shock while measuring. Resistors for this purpose are easily available from Mouser & other distributors. If you choose to use lower precision resistors say 5 or 10 %, your measurements may be less precise. If you go to the trouble of installing these resistors you might as well spring for the 1% version. A pair will cost less than a Starbucks coffee.

[chris61]

I am sure that you are right but The note was on the designers schematics, so I thought i would go there 1st.
I have only a previous posters word that bias is what this is adjusting.
it says adjust it, so i want to adjust it.

[Leo_Gnardo]

I think the designers (probably Pete Traynor) were looking for a fast field measurement that would suffice. We can take our time & come up with something better I'm sure.

Punching the numbers into my handy calculator, Traynor wants you to send 8 milliamps through the pair of screen grids. And they're assuming the tubes will draw an equal current 4 mA, because they're perfectly matched, right?

Something I'd do for starters, is serve each screen grid with its own resistor.

You can stick with the original Traynor plan, but I think you're going to wind up with way more bias current and screen grid current than a currently-made tube will be happy with.

I'm currently setting up a Traynor YBA1A for use with KT88 outputs, and it's been troublesome. Best advice I got so far is that screen grids in modern output tubes of most sorts are happier with about 150V less than plate voltage. First pair of KT88 died a premature death with 535V plates & 531V on the screen grids, in an hour or so. And these are tough tubes, with screen grid rating of 600V supposedly. So much for theory... I'll be concocting another level of power supply @ 375-400V for those screen grids. Maybe the original Mullard EL34 that Traynor typically furnished were able to put up with this kind of screen grid voltage. Me, I can't afford another $80 "experiment" to go bad. Lucky the owner of the amp is being patient & not banging down the door.

[chris61]

I'll put something else into the mix,
I have two of these traynors,
just given to me by a friend;
The 1st we have been taking about, the 2nd is a later mdl Yb1a.
now the thing that bothers me is that the schematic shows 4@40uf filter caps, but 80 uf were installed from the original build,
the schematic seems accurate up to that point, I'm wondering why the change wasn't noted.
that seems like a big difference.

[Leo_Gnardo]

now the thing that bothers me is that the schematic shows 4@40uf filter caps, but 80 uf were installed from the original build,
the schematic seems accurate up to that point, I'm wondering why the change wasn't noted.
that seems like a big difference.

The difference is better filtering of the hi voltage supply. Less hum, and less modulation of your notes when playing at the power limit. Insufficient filtering = "gargling" tone when you play loud. The less of that, the better.

Didn't note the change, who knows, they may have already pasted the schematic on the chassis lids. They could have "penciled it in" but were probably too anxious to bolt those amps together and ship 'em. It happens. Heck there's lots of examples of old Fenders that don't show the correct tube lineup, or even have some other Fender model mentioned on the paper label pasted to the inside of the box.

If you're feeling brave, you could select 100 uF caps or even 220 uF when you replace the hi voltage filters. I went with 220 uF and I'm enjoying the lack of hum. The power transformer is very stout and IMO can stand the change.

Don't forget to swap in new bias filter caps too. Would be a shame to fix up the amp, retube, then have bias supply fail.

[madkatb]

I may be misreading the schematic but where you see 4@40uF I see two at (40 paralleled with 40), which equals two at 80uF. So maybe the twin 40uF caps were replaced with 80uF. (C17 and C19)

[loudthud]

It's not unusual for Traynors to not match the schematic.

The Varistors are usually bad and I've never found a replacement. I usually connect a 270 ohm 10W resistor across the speaker and install ceramic tube sockets for the output tubes.

[chris61]

thats the schematic for the MkII
not the yb1a
let me see if I can post the yb1a

[chris61]