OT upgrade recommendation for SF Deluxe Reverb

[Tone Meister]

'the BASS control is all but ineffective past "3" on the dial'

Many SFs had a bass control with a ~30% taper, same as the treble and volume.
The old BFs generally had a 10% taper; that provides much better resolution / useful control range over the dial.

It didn't become ineffective to this extent until I swapped out the grid leaks to 100K (from 220K). I do believe the BASS control had lost some effectiveness after swapping in those 43K plate loads and then more so when combined with the 100K grid leaks. At this point, I could leave in the 100Ks or increase them some. Bass response is still adequate as is, though, but the BASS control just doesn't have any perceptible affect after "3" or so on the dial.

[Tone Meister]

Pete, I noticed in your AA1113 schematic that you kept the 100K/82K plate loads and reduced the grid leaks to 100K. Do you think that might be a better alternative, or would a reduction of either pair of resistors produce the same result? Also notice the added 100K R at V6 and the 4700 R at the power tubes. What benefit does that 100K add and why did you increase the 1500s to 4700?

I have shows tonight and tomorrow after which I plan to finalize the circuit and fit a pair of Gold Lion KT66 power tubes. The 8-ohm speaker would then connect to the 4-ohm tap so that the KT66s will "see" 8K from the OT. Any other changes needed to the circuit to accommodate the KT66s?

[pdf64]

'I noticed in your AA1113 schematic that you kept the 100K/82K plate loads and reduced the grid leaks to 100K. Do you think that might be a better alternative, or would a reduction of either pair of resistors produce the same result?'
As a single change, bringing the grid leaks down to 100k should be more effective bias excursion / blocking distortion mitigation than using 47k for the plate loads.
I tend to use 5751 in the LTP so the 100k / 82k plate loads seem more appropriate.

'Also notice the added 100K R at V6'

That's a grid stopper to mitigate blocking distortion when the LTP gets overdriven.

'why did you increase the 1500s to 4700'

Again, bigger grid stoppers should help to mitigate blocking distortion.
The above two are theoretical benefits only at this stage, I can't demonstrate any improvement they may bring.

KT66 work ok into 4k or 8k; suggest to try both ways. Depending on the manufacturer, max power is around 4k5 - 6k (my hopelessly unrepresentative sample of 2 pairs, Russian and Chinese).
KT66 from different manufacturers seem to have different ratios of screen grid / plate current. To ensure they're happy, maybe increase the screen grid stoppers to 1k >2W, or perhaps try the JTM45 arrangement of a shared 1k 2W then individual 470 1W. A quick test seemed to indicate that it provides smoother overdrive / more rounded waveform.

[Tone Meister]

With no changes to the circuit (grid stoppers, etc), installed a pair of Gold Lion KT66 and biased to 60% and the amp lost all the character from before. I treid them at 4K and at 8K and there was little to no difference. The result was so uninspiring that I chose not to upgrade the grid stoppers and try to play any shows with the KT66s. In the most simple terms, the amp felt down on power and much too clean - as if the power tubes just weren't being driven hard enough. Since I had (have) 8 shows in 6 days, I decided not to try to make them work in the interest of time.

Put the short bottle Ruby 6L6GCMSTR back in and biased to 60% and life is good. Would like to try the KT66s in the future, but I think I'll need to make some adjustments to ensure they are driven fully.

Make sense?

[pdf64]

Maybe bias them a little hotter, especially at 8k.
I suspect that the 6L6 and KT66 will have a similar gain and move into overdrive at similar settings, but KT66 seem to have a smoother overdrive character than 6L6, which may take a bit of 'aural recalibration'!
But there wouldn't be much point in them not being 6L6 if they sounded the same.
I seem to remember going through a similar process, and now 6L6 can seem a bit harsh to me.

[Tone Meister]

I didn't leave them in very long. My initial impression is the KT66s were much more polite than the 6L6GC, too polite, and a bit lean sounding, and too clean as if the amp was down on power. Maybe I should set up the grid stoppers with the values you suggested.

How hot would you suggest pushing them? I had this pair matched for medium-high current draw (40 mA @ 400V were the matching numbers from Tube Depot)

[pdf64]

I wouldn't bother tweaking the power tube control grid stopper value, I think they would have to be made way higher before there's a noticeable difference.
Maybe set the bias for ~70% 17 watts dissipation.

[Gingertube]

A questiion from above which did'nt get answered.
What does the "A" suffix mean on Hammond Transformers.
It means that the tranny has simplified secondary wiring to enable 4 , 8, 16 selection easily. In the old non "A" suffix trannies they had 2 secondary windings, 1 tapped and one not. You had to re-arrange the secondary wiring connections to achieve the impedance change, some connections had to be made in parallel and some in series. It was possible to use a simple switch to get 4 or 8 Ohms and I think you could also wire it to get 8 or 16 ohms on a single switch but 4,8,16 was damn near impossible without a 3 or 4 pole switch.
The "A" suffix trannies have 4, 8, 16 and common separtate wires out of the tranny .
Cheers,
Ian