Building a 6K6 powered 5E3 from scratch

[Hardtailed]

For months I've been wanting to find a somewhat powerful circuit to use a tranny set I have with. A Hammond 269EX PT (190-0-190 65mA) and 125C OT.

I recently came aware of the 6K6GT, which is not very expensive, and is basically a low-power 6V6! A quick look at the data sheet and they give 64mA current (55 plate + 9 screen) on a 285V B+ with a 12K Z.
Good, I can do that! Should put me at around 70mA current draw including the pre-amp, but with Hammonds being notoriously over-spec'ed I feel safe.
The multi-primary 125C OT allows me to select a 12K Z, good!
My B+ should be somewhat around 265V, close enough. Will also help bring the current draw slightly down as well as keep the OT happy (though, with so many people running the 15W 125E in 18W amps, I 'm not afraid of a 10W amps with the 8W 125C).

So of course, being a baby 6V6, I thought about using this occasion to use the 5E3 topology. Now, it won't be the actual thing, I know that, but it should be fun. For one thing, I will be using diode rectifying since B+ is already rather low.

Now I'm wondering though... do I keep everything as is and thus have the preamp voltages "scaled-down" accordingly?
I noticed the decoupling resistor is a hefty 22k before the pre-amp power node.

I was thinking about going with a 4.7k, like the one between plates and grids?

From what I can see, the 22K resistor seems to be dropping about 100V on a stock 5E3. With my low B+, I could end up under 150V on the preamp tubes.

How low can I safely go with the decoupling resistor without creating noise or oscilliation issues?

What about running a separate node, parallel to the screen one instead of in serie, to get less voltage drop?

Thanks!

[tubeswell]

Remember that the voltage drop across the supply resistor will increase as you decrease the input voltage (with a fixed resistance). So maybe try building it with a 10-18k (instead of 22k) and tweak it from there to get the right pre-amp voltages

[Hardtailed]

Originally Posted by tubeswell

Remember that the voltage drop across the supply resistor will increase as you decrease the input voltage (with a fixed resistance). So maybe try building it with a 10-18k (instead of 22k) and tweak it from there to get the right pre-amp voltages

I was roughly scaling it down in the same proportions.

The chart I've seen was showing 390V on the plate, 339V on the screens and 242V for the preamp.

If I start from 265V, that gives me 230V on the screens and 164V for the preamp node. Just feels kinda low

[tubeswell]

Originally Posted by Hardtailed

I was roughly scaling it down in the same proportions.

The chart I've seen was showing 390V on the plate, 339V on the screens and 242V for the preamp.

If I start from 265V, that gives me 230V on the screens and 164V for the preamp node. Just feels kinda low

A 6K6 doesn't need as much juice as a 6V6, but it still needs around 280- 320V on the plate

6K6GT @ The National Valve Museum

So I think you'll have to go for a PT with about 220-0-220 to 240-0-240 on the HT VAC if you're wanting SS rectification (or 280-0-280 with a 5V 2A winding if you want a 5Y3Gt rectifier)

[Hardtailed]

I understand this is not ideal, but this is more a case of trying to reuse parts I already have on hand.
See, I built this 1W Firefly tube amp last year using a PCB, and I'm just not finding it very useful. Still too loud to push at home, yet too quiet to use with the band (I carry it around as a spare, but it needs a lot of help from the PA!).

Running a pair of ECC99 in parallel would allow me to get around 5W... but I just don't like triode power tubes. Sounds too "boxy".
I'm really trying to reuse as many parts from my Firefly as possible as I'm strapped for cash at the moment.

So I decided to go ahead and try 6K6GTs.

Based on this: TDSL Tube data [6K6-GT]
The following Application is perfect for me:
A P/P 285 285 55.0 9.0 400 shared 12,000 9.8

I'm thinking that between 265V and 285V, not such a big difference (not to mention mains are usually a bit high around here so I can expect over 270V). I can expect slightly less output, probably around 9W, good enough for me and within the limits of my transformers.

So I'm rebuilding the whole amp in a new chassis, using the 5E3 topology instead of the Firefly, except with the 400ohm cathode resistor and diode rectifying.
I will be allocating space for an extra socket on the chassis, so that if I ever decide to change it to a full-fledged 5E3, I can pop in the tube recto along with putting in new transformers.

Should be fun to see what the end result sounds like.

As for my fears of too low preamp voltage, I noticed that the Firefly actually has TWO 22k decoupling resistors and still has over 200V on V1, so I'll go with this at first. I can always parallel a 2nd one if I find B+ too low on V1 and see what that does. Easy mod.

The Firefly has a 12AU7 as the power amp, so I will be trying this in the V1 position, I've read good reports about it here (didn't bother ordering a 12AY7 for now, but I can find one locally easily).

[Hardtailed]

Just an update: it's up and running!

I've got an odd selection of 6K6s: 2 Westinghouse, 1 GE and 1 RCA!
Fun stuff.

I went with 12K primaries and 400ohm for the cathode resistor, as per the tube datasheet.

Sounds so much fuller than when it was a Firefly, confirming the transformers are not so bad!

It's fun experimenting with the interaction between the channels.
Although I have some buzzing problems right now, but I only have my bass on hand and I think it has some shielding issues. I've reworked the grounding scheme but to no avail. Can't wait to try it with my guitars.

As expected, the preamp B+ is rather low, but I noticed that using a 12AX7 instead of the 12AU7 (I don't have a 12AY7 on hand) brings it back up a bit since it drops less voltage on the decoupling resistors by pulling less current.
The different is dramatic, more "kick" to the tone and more headroom.