SET amp from 1619 driving a 809. Opinion wanted.

[rcavictim]

I have had a need for a flea power, very high quality stereo power amp now for the purpose of evaluating my high efficiency full range loudspeaker designs for suitability of working with customer's 45, 2A3 and single 300B type SET amps in the marketplace. I wish to be DC coupled and employ only DH tubes for their alleged magic tone. DC heat will be employed throughout.

Since my inventory of tubes is not blessed with such valuable gems as 45's or 2A3's, 6B4's, or DH input suitable tubes of conventional choice I have decided to be creative from what I do have in stock.

Following the general theme of the famous WE 91A amp that used a pentode to drive a 300B, I have decided to use a 1619 as the line input stage and driver. This transmitting beam pentode tube is similar to a metal 6L6 with a 2.5 volt thoriated tungsten DH cathode. I have 11 NIB, NOS. I was torn to use one also as the output tube in triode mode and still may. This would be about the equivalent of a 2A3. I am however looking first to use an 809 power triode because of the esthetic appeal. With so much effort being put into making this amp look like art (not being able to just throw stuff together is a curse of mine), I would like to see the friendly, soothing, reassuring glow of DH valves. The reward for all this effort!

My circuit directly couples the driver plate to the output tube grid with a plate choke on the driver (reactance drive). My concern is what will happen when the 809 gets into A2 operation. I don't know how well it will perform kept in A1 land only. Unfortunately I have no interstage xfmer suitable to drive the 809 into A2 properly and I cannot wire the input tube as a follower as I need the stage gain and do not wish to go to 3 stages.

Attached is a picture of the layup so far. The top plate is polished 3/32" yellow brass under the protective paper. Wood is oiled black walnut.

Has anyone employed 809's in SET, especially A1? What were your results please?

[Gluca]

Curious about yr project ... What's the 809's op point & OPT? I thought it would like to be run in A2 only.

Ciao
Gianluca

[rcavictim]

Yes, I am worried that it may not work in A1 that well. I am using one or other of a 15 watt Hammond vintage stock P-P transformers. I have a set of NOS each of 1610 (10K CT and separate 4 ohm and 16 ohm speaker windings) That gives me options of half or full primary and using the 4 ohm winding in cathode NFB of the 809 if I wish. 16 ohm to 8 ohm speaker load which would give me 5 K or 2.5 K on the anode.

The other two xfmers I have are 1615. Those are 5 K CT and have 4-8-16 ohm taps. I can get a large variety of AC plate load options with these from 2.5K to 20K.

I am parafeeding the ouptut xfmers with a series motor run cap so no DC in the OPT core. A plate choke is used. The coupling cap is also to be connected in ultrapath. I have an idea of running about 40-50 mA on the 809's at about 470-500 volts plate to cathode (~20-25 watts PD.) The tube is rated at 25 watts CCS.

The 1619 driver sits underneath the cathode of the 809 and will see about 250 volts plate supply through the 809. Total B+ should be up around 700-750 volts. Choke input filter for good regulation.

Planning for possible failure, if the 809's don't work out for me I can plop in 811A's and dink around with those, or 300B's with only minor changes to the filament supply and bias resistors. Real SE output iron can also be acquired eventually if the amp looks to be worth spending money on and performance is not yet stellar.

This project is all from the junk box for now except for some big value, low voltage electrolytics I had to order for the DC filament supplies. Shielded filament xfmers are custom by your's truly.

[Gluca]

But that point seems to be at +7V grid-to-cathode and grid DC is really flowing there



I would use an IT. Same with 811A.

Or to stay on the cheap side, I'd use that follower you don't want to add ... with choke in the cathode. Yes that's a 3 stages amp.

Well ... very little help from me...

Gianluca

[rcavictim]

The case was made with the largest pieces of walnut I could salvage. Initially I wanted an 11"x17" chassis. This one is just 9"x10". The circuit is dictated by the amount of wood I had and available parts. No room for two more tube sockets I'm afraid. A pair of IT's could be fit underneath if I make room by getting rid of plate chokes for the ouptut stage. To do this I'll need to buy SE output xfmers for topside. That will cost money. So will IT's. Not an option just now.

This whole thing is just an experiment. If tweaking doesn't work, then it certainly can be made to work when I can throw some parts money at it.

I guess I'm into this far enough now that I'll know the answers to my questions soon enough finding out by myself.

[tubelab.com]

I have some 809's but have never tried to use them. I have tried 811A's. Two stage amp. Same circuit as the TubelabSE. They work good. I use mosfet followers to supply all the grid current that any output tube can eat.

[rcavictim]

Took me all day but I now have the brass top plate machined and overcoated with clear lacquer. A second coat of linseed oil is drying on the base and tomorrow I can start assembly.

I changed formation from the flying Vee (the tube location pattern first considered) and moved the electrolytic caps in between the rectifier and the output bottles. This should reduce the possibility of induced rectifier noise by more distance and grounded obstacles, and gives me more room to accomodate other output xfmers later on just in case.

I will need to make sheet metal sub-chassis for underneath. I want to be able to lift the entire circuit out of the case by the top plate and not have guts screwed to the insides of the wood. There is just no room to work in there that way. This thing is going to be well packed.

[rcavictim]

Originally posted by tubelab.com
I have some 809's but have never tried to use them. I have tried 811A's. Two stage amp. Same circuit as the TubelabSE. They work good. I use mosfet followers to supply all the grid current that any output tube can eat.

Thanx Tubelab. That is an idea that hadn't crossed my mind. I'll consider that as a backup plan if I hit a dead end.

[rcavictim]

Here's the compulsory tubes shot.

[Gluca]

well ... that's really nice. Envy here.

A good idea from Tubelab ... it seems it won't take such a big room.

Gianluca