845 SE Power Amp

[Gingertube]

I do some restoration/repair work for a local professional muso (lead guitarist).
Last time he was around my place he spotted an 845 Triode sitting on my shelf and immediately asked for an guitar amp to be built with "one of those".

So I sat down to do the design
Preamp is done but now am looking at the 845 Power Amp Section.
I'm going to run Hammond 125ESE (15W rated) at around 20W Out using around 850 to 900 Volt Rail.
Tossing up between a 6V6 to drive the 845 (240V pk-pk grid drive Class A1) or a 1:3 stepup interstage tranny drive (I have that in stock as well).

Has anyone done this before?
Anyone offer advise on the above methods of driving the 845.

Also I'm a bit concerned that it is going to be VERY clean, perhaps too clean for a Guitar Power Amp - opinions please?

I've dealt with my main mechanical /reliabiltyconcerns by putting the 845 on a shock mounted sub-chassis.

Thanks,
Ian

[Chuck H]

I'm finding limited data on the 845. This is an idea who's time has come though. I've often thought about a giant "Champ" as it were. I wasn't able to find any info on grid current requirements but I think your plan to allow for it is a good one being as it's a triode capable of obscene power. The grid bias voltages and grid drive voltages are big enough, eh? No reason it needs to be too "clean" if you can provide the drive it needs. Looks like you'll need more than 60V AC on the grid at your Vp to get it clipping though. Tall order but a fun challenge. Sounds like a very satisfying project. Looking forward to the thread.

[Gingertube]

Chuck,
I have some (limited) experience with the 845 from a Chinese "Music Angel" HiFi Amp.
With 900V Vak it biases (80mA idle) at around -110 to -120V. Original RCA 845 had an anode dissipation limit of 100W but I've never been game to run the modern Chinese 845 at more than about 80W.
I have a friend in Sydney who runs Parallel 845 in A2 for 75W Out in his HiFi so will also "sound him out" so to speak..
You can get 20 to 25 Watts out of a single tube reasonably easily in A1 (No grid current) although for higher power it likes to run up around 1250V.
With regard to grid current it (the 845) is quite unlike 813, 805 and other high power "transmitter" triodes which do need significant power delivered into the grid.
I basically settled on the 900V max 20W output so that I could use the 125ESE AND so that I can get away with 3 off "standard" 450V electrolytic caps stacked for the power supply
The 845 are not that expensive - a pair can be purchased for around US$100 - $120).
Expensive enough that I did'nt begrudge the $15 each for the four shock mounts, if it saves one tube falling apart then they've paid for themselves.

Cheers,
Ian

[Gingertube]

THe 845 Data Sheet.
http://www.pentalaboratories.com/pdfs/845.pdf
Seems like I will be running at:
Va = 900V
Ia = 72mA (65W Anode dissipation)
Vbias (Vg1) = 140V
Peak AF signal to G1 = 135V so driver will need to be able to swing 270V pk-pk minimum.
OT primary impedance 8K
That should give 20W out at 5% THD.

Note the heater requirement 10V @ 3.25A = 32.5W of heater power - no wonder these things heaters don't just glow red but light up bright yellowish and look like a light house.

Like all SE circuits, not very efficient, 32.5W heater and 65W anode dissipation (over 100W total) to get 20W out.

Won't be starting on it this weekend - I have a "spiff up" job (noise reduction) to do on a "Junkbox Trainwreck" I built to do, ground the pot cases and float the heaters on +70V DC should fix it.

Cheers,
Ian

[Chuck H]

How much drive voltage do you think you can manage? It would be nice to get it clipping.

You should build it so the top of the tube sticks a hole on top of the head and is encase it in a wire cage... Maybe a big analog meter on the front panel monitoring current. Serious cool factor.

[Gingertube]

I can manage more than 270V pk-pk. Just a matter of getting the design right.

Do you have a feel for how much overdrive I should aim for. You are quite right that I need to be able to drive it into clipping.

Brain Storming:
Extreme case - An EL34 as a driver running at 500V should be able to swing 440V Pk-Pk (assuming a Vsat of 60V)
Similarly a 6V6 at 400V would manage 360V pk-pk (Vsat = 40V)
I can probably manage 300+V pk-pk from a SRPP driver. Since I'm not trying deliver grid current I could probably use a 6SL7. A 6SL7 SRPP would have a gain of approx mu/2 = 35 so would still need another gain stage ahead of the SRPP
The 1:3 interstage tranny I have will (I think) not cut it - needs an experiment

If I were doing the full vintage (expense no object) number I might even consider getting a more suitable interstage tranny in and drive it with a 2A3 or 300B BUT I have to keep this affordable AND we all know that guitar players (or musos more generally) never have any money. Otherwise I'd do this for a living instead of as a hobby (So I have a day job and design Laser Airborne Depth Sounders for Hydrographic Survey instead).

Many ways to skin the cat.

A friend in Sydney does this in his PSE845 HiFi Amp
SRPP => current source loaded common cathode stage => white cathode follower => parallel 845 for 75W SE from a pair of 845 at around Va = 1050V
I could just be lazy and copy that but it is a HiFi design.

I'm leaning toward the EL34 or 6V6 "Brute Force and Ignorance" approach as being best for a Guitar Amp (best as in good enough).

On the mechanics - The 845 sits on a sub chassis about 30mm (1-1/4") below the main chassis with all but the tube base sticking out the main chassis top.
A heafty cage will be essential to protect the tube from mechanical knocks, children and small animals and to protect the children and small animals from the tube.
With 100+W dissipation from the tube it gets HOT!!!!

When I was running the 845 HiFi Amp the cleaning lady melted her duster on one of the 845s.

Cheers,
Ian

[Chuck H]

When I was running the 845 HiFi Amp the cleaning lady melted her duster on one of the 845s.

Brilliant.

I say go the 6V6 route. Even if the grid "blocks" a little that's still part of the sound for a guitar amp. Remember that there are certain anamolies that are accepted (or even expected) in the distortion character of a guitar amp. The 6v6 should prove effective and affordable (though I've imagined an interstage tranny in this application too)

[Gingertube]

Will do - I have some nice Sydney Australia AWV Factory 6V6G (Coke Bottle Shape) from 1950's, a heap of RCA 6V6GT and some 1980 Russki 6V6 (GT) which still had the KGB stickers on the box they arrived in, so plenty to try and choose from. I must have 60 or 70 6V6s all up AND they are my fave tube (that is just the ones that remained after my entire tube collection went thru' my AVO MKIII tester, I don't keep anything which doesn't test up to scratch). I'm running them in my HiFi Amp and my current Guitar Amp.

Cheers,
Ian

[Ronsonic]

Don't worry if it's clean. The world doesn't need more grind, well it does but not every amp has to do it. I'd come up with a driver with plenty of headroom and go for sweet and clean. IINM the 845 will take being driven positive into class A2 if your driver can handle it.

[jazbo8]

Will do - I have some nice Sydney Australia AWV Factory 6V6G (Coke Bottle Shape) from 1950's, a heap of RCA 6V6GT and some 1980 Russki 6V6 (GT) which still had the KGB stickers on the box they arrived in, so plenty to try and choose from. I must have 60 or 70 6V6s all up AND they are my fave tube (that is just the ones that remained after my entire tube collection went thru' my AVO MKIII tester, I don't keep anything which doesn't test up to scratch). I'm running them in my HiFi Amp and my current Guitar Amp.

Cheers,
Ian

Hi Ian,

Did you ever build the 845 amp? Schematic & comments are welcomed. Thinking about a "Big" SEL (AX84) design...

Jaz

[Gingertube]

Jaz,
Not yet - about to start when a friend dropped in a pair of Aranov LS9100 100W HiFi monoblocks to fix for him (he let the smoke out of one of them), and another dropped in a vintage Vox AC30. It seems everytime I clear the decks to start on one of my own projects some one turns up with a job to be done first.

Cheers,
Ian

[jazbo8]

Ian,

Too bad, I saw your post on the Aranov - I think you have created the only known copy of the LS9100 schematic in existence! Those were damn expensive amps, even second hand... Debating whether to go for 845 in A1 or 805 in A2 (using PowerDrive from Tubelab), your suggestions and comments are appreciated.

Thanks,
Jaz

[Gingertube]

Jaz,
I thought through a few of the issues and for what it is worth these are my thinking.
1) You can get plenty of power from an 845 without resorting to A2 operation (grid current) so lets not and that immediately simplifies the driver as we are no linger delivering power to the grid.
2) Next is really to choose an opearting voltage for the 845 (anode to cathode voltage) Options are:
- approx 800V, using 8K primary impedance for around 20 Watts (Hammond 125ESE Output tranny will do). Complication comes from needing 3 off caps in series in the power supply to handle the peak voltage at switch on, before the 845 starts to conduct and pull the rail down a bit. 2 x 450V caps in series can be used if you implement time delay relay switching of the High voltage.
- approx 650V, using 5 or 6K primary impedance for around 12 to 15 Watts, voltage is low enough to use 2 x 450V caps in series for the power supply without any fancy timedelay switching etc., agaimn the Hammond 125ESE will do.

Probably cathode bias for simplicity, if going fix bias you will need a -150 to -200V supply (bias is typically around -120V with anode to filament of 800V).

For the driver - we need to swing +/- 120V (240V pk-pk). Parallel 6SN7 sections will do that or use another power tube, most have too low a screen voltage rating to try triode strapping so pentode mode 6V6 is probably what I will use, Triode strapped KT77 would be an option, 6L6 or EL34 in pentode or prabaly also triode mode would do and if you don't mind mixing miniature tubes with the big boys then EL84 would be OK even if it will look wimpy next to the 845.

I have attached the schemo from the 845 HiFi Amp which may give you some ideas. It uses a 6SL7 (6N8P)SRPP front end into a 6SN7 (6N9P) driver, fixed bias 845

Cheers,
Ian

Cheers,
Ian

[jazbo8]

Ian,

Thanks for the feedback, I jumped the gun a bit and got myself a 805 (actually a Chinese FU-5) since it is significantly cheaper than the 845. Since this will be my first amp that have B+>400V, I want to reduce the complexity a bit and forego the bipolar supply which the 845 requires. I am using a known working design to save me the embarassment and the associated risk of messing up on such HV design. It's just a simple SE design but in A2 as the 805 prefers. In the schematic below, you will notice the cathode choke (45H, 360R) which is the key to make this rather simple design to work, here is the simulated grid signal of the 805 at +-40V.



Its designer has a write up at 805 Amp Design Analysis, even though google has completely butchered the translatation, you can still make out what he was trying to say, he also posts sometimes at diyaudio, I think that's how I came across this particular design. Anyway, I am not going to mess with it other than to change the front end to guitar gain stages. Again, your comments are appreciated, it will still be a few weeks before construction begins, as I am still gathering the parts, particularly the irons required.

[Gingertube]

Jazbo8 etc.
Resurrecting this thread, as I have finally got around to starting this project. Ordered a 1:3 step up interstage tranny today to drive the 845.

Would you suggest keeping a "typical" vintage SE Guitar Amp front end, that is, a 6J7 or 6SJ7 pentode to a 6J5 (or triode strapped 6J7 or 6SJ7) or copy a Champ?

Cheers,
Ian