Hi-Fi Techniques for 'Acoustic' Tube Amp?

[jbefumo]

Did search and saw the various old topics, which I will be reading in more detail as I move forward, but thought this might be a somewhat different slant.
I have the OT and OT from an old Sundown that made 100W from a pair of 6550s, along with a lovely 1X15 blonde cabinet and a cherished Altec Lansing 418, a 15" guitar speaker with a claimed response of 45Hz to 8000 Hz.  I've had it in a number of amps and it's hands-down my favorite speaker, offering a remarkable sparkle without the harsh edge that some JLBs have.
Been dabbling with a modded Tweed ODS board in with these parts, using a pair of Gold Lion KT88s, but that's way more than I need for electric, and as I'm increasingly getting into acoustic, I was thinking this might be a good application for that platform.
I was thinking I might invest in Merlin's new book, Designing High-Fidelity Tube Preamps, and am wondering whether that would be a reasonable approach to designing the preamp circuit.
An added bonus would be if it could also serve as a passable bass amp.
 

[PRR]

I would not use the 418 as an electric bass speaker at the 100W level. It will take full-range fine, but not designed or built for thump-only.

No clear thoughts on acoustic guitar. However instrument pickups are lower-output than hi-fi gear, so if you study Merlin you will need changes.

[jbefumo]

Oh, not going to keep it at 100W -- I think that was pushing to begin with.  Was thinking I would shoot for around 60 tops. Actually had that speaker in the Sundown and it worked great (for guitar), but I never really pushed it that much.  May go for a 5V transformer and use tube rectifier, and/or cathode biasing, though I think I might have an issue with the OT primary impedance. A bit hazy on that end of the circuitry ...

I would not use the 418 as an electric bass speaker at the 100W level. It will take full-range fine, but not designed or built for thump-only.

No clear thoughts on acoustic guitar. However instrument pickups are lower-output than hi-fi gear, so if you study Merlin you will need changes.

[PRR]

> I have the OT and OT

I ass-ume this meant "PT and OT"; that you have an iron-set which was professionally selected to work together.

> or cathode biasing

40W is about the limit for a 35/42W Pdiss tube in self-bias.

Datasheet shows for 5K loading 40W at 400V and 100W at 600V. Reasonably there will be a 60W point near 500V DC supply. 450V-550V will all be too close to care.

If the original was Silicon rectified, *and* the PT is CT HV winding, then yes a vacuum rectifier would drop a good amount on voltage and power.

[jbefumo]

Yup -- PT and OT is what I meant.

> I have the OT and OT

I ass-ume this meant "PT and OT"; that you have an iron-set which was professionally selected to work together.

> or cathode biasing

40W is about the limit for a 35/42W Pdiss tube in self-bias.

Datasheet shows for 5K loading 40W at 400V and 100W at 600V. Reasonably there will be a 60W point near 500V DC supply. 450V-550V will all be too close to care.

If the original was Silicon rectified, *and* the PT is CT HV winding, then yes a vacuum rectifier would drop a good amount on voltage and power.

[jbefumo]

O --- yeah -- no CT.  What I was thinking of doing was a hybrid arrangement with two diodes and the tube rectifier making a FWB -- will that not drop at least some of the voltage? Of would I just be better off staying with all solid state?

[snip]

If the original was Silicon rectified, *and* the PT is CT HV winding, then yes a vacuum rectifier would drop a good amount on voltage and power.

[PRR]

> a hybrid arrangement with two diodes and the tube rectifier

That works.

(But do you have a separate 5V 3A winding? At this power level, none of the insulated cathode rectifiers are really suitable.)

[jbefumo]

Have a separate 5V transformer on the chassis at the moment. (It's the one from the bottom of this page:


http://hoffmanamps.com/MyStore/perlshop.cgi?ACTION=enter&template&thispage=Transformers&ORDER_ID=!ORDERID!)

> a hybrid arrangement with two diodes and the tube rectifier

That works.

(But do you have a separate 5V 3A winding? At this power level, none of the insulated cathode rectifiers are really suitable.)