Mini-Major build

[Matt T.]

Hi All,
I've been away for a while but I've got the bug to build again. This time it will be a mini-Major build but using a Hiwatt chassis, boards, and layout. I'm getting transformers from Merren Audio, he makes a 2xKT88 mini-Major OT and then one of his 100W Marshall PTs. I intend to install a switch to go from stock to the 'one-wire-mod'; this is something I took from an EL34World thread from 2014-ish. fusionbear on TGP recently built one too. The uber clean Major circuit appears to work well with the OWM.

Found a new set of Gold Lion KT88 for $75. I've heard this is probably the best new production option. What ballpark bias voltage should I be looking for if my plate voltage is 500-ish vdc?

Any warnings or other sage advice appreciated. I intend to use separate 270R/5W for the screens (U/L connection). The old Major schematic shows 100R resistors for each plate and 39R resistors for each cathode but the newer schematic does not have them so I was going to omit them. Why are they there in the first place?

[KevinOConnor]

Hi Guys

Bias voltage is not what matters, rather it is idle current and idle dissipation. The value for your amp depends on the plate voltage and the dissipation rating of the tubes.

KT-88s are rated at 42W and your B+ will be low compared to a real Major, maybe around 500V say. Every 10mA through the tube at 500V corresponds to 5W of heat pouring off the plate. generally, any idle point between a bit above zero watts and the full rating of the tube is acceptable. Yes, the tube can dissipate its full rating all the time, which is why it is rated this way. The exact idle current you choose, and thus the exact dissipation, depends on the tone you want. Colder running provides a drier tone that may lack sustain, but too-hot may not provide any benefit compared to a median setting where the compromise between heat and tone is usually pretty good.

The above is just Ohm's law. So, at 500V, the 42W limit imposes a maximum idle current of 84mA, at 600V this would reduce to 70mA but at 400V would rise to 105mA. In all three cases, the heat output is 42W.

Where the bias voltage is the MOST IMPORTANT voltage in the whole amplifier inasmuch as it keeps the tubes from melting down, its actual value is unimportant. There should be sufficient range of control to be able to turn the tube off so that you have control over how hot it idles. The universal definition for over-under bias is related to HEAT. In my amps and kits and books, I use the X-end (CW) of the bias pots as the hot end corresponding to high bias, with the 0-end (CCW) as the cold end or under-bias.

In general, KT-88s and 6550s need a bit more control voltage than other tubes, but as always, it is based on the screen voltage. If Vs is essentially the same as Va, then Vg1 (absolute) might have to be about 15%. For 500Vs then Vg1= -75V. There should be much more than this voltage available for proper control. If you are using the UL connections, remember to use 1k screen-stops. Were Vs=600V, then Vg1 would have to increase as well to about -90V.

The original Major had these higher voltages and thus the higher drive requirement which is why the paraphase stage was added to drive the output tube grids. I built a more mini-Major back in '89 using a Traynor YBA-1A as the foundation. I found the EQ rather lack-luster. As I noted in another thread here, the UL output stage sounds fantastic when used open-loop, and you might want to experiment with that once you have everything functioning.

Have fun

[Matt T.]

Hi Kevin! I still have one of your Power Scaling circuits in an amp I built quite a while ago. Thanks for the detailed response.

I should have stated the nature of my question. Since this is a new build using a new PT, I was concerned whether the existing Major bias circuit would get me in the right range for KT88s at these voltages. I know the basics of calculating dissipation, current, etc. I'm just looking for a ballpark number; like am I looking for ballpark of -35v? -45v? -55v? -65v? When I initially fire up the amp, I want to have sufficient bias voltage such that any set of KT88s will be cold, then I'll adjust hotter from there. External bias test points and adjustment pot of course.

I do intend to run it U/L. Since the original used 250R/5W SGRs I was going to use 270R/5W. If 1K/5W is that much better without compromising tone I'd rather use that value. But I've increased SGR values in other amps and have always gone back to the lower value. But those were 4xEL84 cathode-biased amps and even though the 100R value there is pretty low, I thought it sounded best. And not ALL of the AC30s caught on fire.

[KevinOConnor]

Hi Guys

The 15% target is a good one yet most amps never attain it.20% is even better and will definitely turn off all samples of tube inserted unless the tube is defective.

Higher screen-stops are especially important in any EL-84, EL-34 or high-bias amp. These tubes and conditions are prone to thermal runaway.

Have fun