Standel 25L15 clone PT recommendation

[xk49w]

I've been doing some simulations of the preamp. The network driven by V1a, the tee-like affair is a band-reject filter. The center of the notch is 200 Hz, depth is about 5-6 dB below the gain at 50 Hz. That network has a generally rising frequency response with about a 12 dB gain increase from 50 Hz to 6 kHz.

The active filter is a boost-only network capable of about 10 dB of bass boost and 15 dB of treble boost. With both treb and bass at zero it's flat with a stage gain of around 20 dB. Total preamp gain from V1a input to the grid of v3b is 35-40 dB with tone controls flat.

[img:895:537]http://lh5.ggpht.com/_jpUbIh2UclM/Sywcn ... c_gain.jpg[/img]

[img:893:538]http://lh4.ggpht.com/_jpUbIh2UclM/Sywcp ... e_gain.jpg[/img]

[Andy Le Blanc]

This is an old Peavey scheme, VTA 400, might not seem related but it has
several text book examples how and where active tone controls can be used.

Its nice to see so many examples in one place, nothing new, not reinventing the wheel.

The older appearing Standel scheme does something a little different.
A very simple tone control over whats going to be applied as feedback.

But... look at how its applied over cathode of the first gain stage.
That's pretty unique for a guitar amp. In an old radar manual I found once
it was refer'd to as electronic mixing, as opposed to resistive mixing or capacitive mixing.

The closest thing to that is how some tremolo circuits are coupled to the
pre in some old lower end amps, it'll be fun to noodle with.

[Andy Le Blanc]

"similar to Langford-Smith page 646"

Do you have a scan, link to a pdf, or a more complete bibliographic reference?

[FYL]

Do you have a scan, link to a pdf, or a more complete bibliographic reference?

I believe it's Radiotron Designer's Handbook, Fourth Edition, aka RDH4, available on-line from Pete Millett's excellent site as well as others.

http://www.pmillett.com/Books/RDH4.pdf

I've checked my printed copy and pp. 646 shows a bass control in the NFB loop between the anode of a 6V6 and the cathode of it's 6SJ7 driver.

[xk49w]

"similar to Langford-Smith page 646"

Do you have a scan, link to a pdf, or a more complete bibliographic reference?

[edited - FYL got in there before me - duplicated info deleted]

Yep. That's it.

[Andy Le Blanc]

Excellent..... thank you, every one needs good reference....

The pre looks to be an easy build, even point to point....

Now the power side uses 807, I keep seeing stated that 6l6 have the same characteristics.
The 807 have the plate connection brought out of the top of the tube.
A high volt clip and lead might be intimidating to some, The plate connection
is that way, as a transmitting type, to keep the grid to plate capacitance low.
A 6l6 does not. The max ratings are different. The 807 can be pushed to a 600v
plate voltage. You can get 80W out of a pair of 807, 600v plate, 300v screen
and a 6.6k load. The max plate dissipation is 25w, which is better than a
6l6,6l6-G or 6l6-GB. 6l6-GC has a max 30w plate dissipation, all share a
similar structure as a beam power tube but the grid no.1 to plate capacitance
of the 807 is half that of a 6l6, which probably contributes to a unique tone.
Besides, a 6l6 doesn't have a cool ceramic base.

[FYL]

Now the power side uses 807, I keep seeing stated that 6l6 have the same characteristics.

They are closely related, but the 807 uses a different screen geometry and can be prone to parasitics.

Max screen dissipation shouldn't be exceeded : either large screen stops or a well regulated supply should be used.

A 47R or 100R resistor or a 20µH choke should be inserted in the plate circuit as close as possible to the plate.

This done, the 807 is tough as a boot...

The 807 have the plate connection brought out of the top of the tube.

The phenolic bases then used weren't qualified above 450V, using a dedicated plate connection allows for a 600V (CCS, Continuous Commercial Service) / 750V (ICAS, Intermittent Commercial and Amateur Service) plate rating.

STC has published a very comprehensive application report on the 807, nearly everything is there: http://www.retrovox.com.au/STC807.pdf (largish file at 5.7M or so)

If you fancy triode operation (the 807 is really excellent in this mode), RCA describes a nice set of params giving 30W for a PP with a 400V plate supply: http://greygum.net/files/radiotronics/n ... itions.pdf

Besides, a 6l6 doesn't have a cool ceramic base.

Most come with a low-loss Micanol base.

[Andy Le Blanc]

I can see this circuit as being more than adequate with several power side
configurations and power tube types. For a clone, it would be best to measure
an opt for turns and an impedance ratio, get a better definition of the load
that is to be presented to the 807's, see how the opt is being used.
It could be that the speaker is purposely mismatched to arrive at a more
optimal plate load reflected upon the primary.

You can do it by the existing data for the tube type, but with no data on a
working example its a shot in the dark, for a clone that is.

The possible Weber choice was a "WOT45UL", any data?

[FYL]

The possible Weber choice was a "WOT45UL", any data?

Probably a modified WOT45, with UL taps.

[img:646:572]https://taweber.powweb.com/store/wot45sch.jpg[/img]

[tictac]

The Schematic Labeled ST8 has the Plate Voltage at 455V and the screens at 462V. Seems really out of wack. The screens of the 807 are supposed to have a max voltage of 300V.

Are those actual voltage readings out of a Standel amp?


TT

[FYL]

Are those actual voltage readings out of a Standel amp?

I don't think so. The PT puts out 350V, the valve is a 5U4G and the filtering is CLC, plate voltage is 360V at most.

[xk49w]

Other signals brought in on a 6-pin connector. Probably a Hubble like Leslies use.

Wrong. Checking the VinPreAmp schematic, two connectors are shown:

1. Power, switching, ground - Jones P-306-CCT
2. Signal out - Jones P-303-CCT

Cinch makes them. Both still available at the usual places.