Tubelab SE in parallel?

[Mazuki]

I'm contemplating building a 300B PSE amplifier. The Tubelab SE design looks really good, especially with the solid state hybrid setup.

The original design is only for one output tube. In order to parallel the output tube, aside from doubling the power supply current and making sure the output transformer is within rating (2.5kohm load ok?), do any other component values need to be changed?

Thanks

[Evenharmonics]

Originally posted by Mazuki
especially with the solid state hybrid setup.

What do you mean by that?

Are you trying to build a monoblock Tubelab SE?

[Mazuki]

quote: Originally posted by Mazuki especially with the solid state hybrid setup. What do you mean by that?

It's got the mosfest in the interstage instead of a transformer

I'm trying to make it PSE, two 300B tubes per channel

[JPeitzman]

300B PSE's that I have seen use 1k5 or so output transformers, though this isn't set in stone.

I am confused though at what you are doing. I fully understand what you are trying to accomplish, however you cannot just change some component values and some iron and run this amp PSE. The Tubelab SE board is setup for stereo, so at the very least you will need two boards. You will also need to do some surgery (not recommend) to bridge some traces to make it PSE. Why not just find a suitable PSE schematic and build it instead?

Cheers

James

[Mazuki]

I am confused though at what you are doing. I fully understand what you are trying to accomplish, however you cannot just change some component values and some iron and run this amp PSE. The Tubelab SE board is setup for stereo, so at the very least you will need two boards. You will also need to do some surgery (not recommend) to bridge some traces to make it PSE. Why not just find a suitable PSE schematic and build it instead?

If needed, I can air wire another tube socket to the existing board. I haven't exactly decided whether or not to stick to a PCB.

I've looked around at PSE designs and I've more or less settled on this one. Andrea Ciuffoli's design looks good but I'm not so keen on using an expensive interstage transformer nor the extensive choke filtering in the power supply.

[JPeitzman]

A design like his (Andrea's) wouldn't be that much more, if not close to the same price to implement. The Lundahl that he uses is about $115, an IT from Electra-Print (which I would venture to say is equal quality) is about $25 less. You would be eliminating the CCS, FET follower, a high quality coupling cap ($$$), and a hand full of passives... I would venture to guess (if a very nice coupling cap is used) close to that $90 give or take. Obviously if you are using orange drops and generic passives then the Tubelab wins hands down.

Either one is a great design, though there is something that draws me towards the Tubelab. Also you don't need that complicated and expensive of a PSU. A CLC would be fine with a RC if you need to drop more voltage and an RC for the front end. Chokes from Electra-Print are about $34 and Hammonds are about $20 or so, it wouldn't be that expensive if you wanted to go CLCLC.

Personally if the shoes where on my feet: 5842 > Electra-Print IT > Parallel 300B's. Stick an LED or string of LED's on the 5842 cathode for bias (eliminate the k bypass cap) and use fixed bias through the secondary of the IT for the 300B's (again eliminate a k bypass cap and a resistor). Use a C-L-C power supply, nailing the B+ by using a custom power transformer (Electra-Print) or by tweaking the first cap, with a pair of 6AX4GTA's for the rectifier. Instead of an additional R-C section for the driver I would use a CCS loaded shunt regulator, CCS made from depletion mode FET's and the shunt from either FET's or gas regulator tubes.

If you stick with the Tubelab design I would make sure to get in touch with George about what you want to do. Not that it wouldn't be possible, I just think it is more work than needed. The Tubelab design can be done point to point too so you don't have to cut up traces and hard wire on a PCB. Also George has a good idea about the limitations of his designs, make sure the board can drive parallel 300B's OK. Just my 2 cents

Cheers

James

[Mazuki]

Well, I was going to use the LM-9907P from VT4C.com and probably some choke from there. So the prices should be lower. I already have teflon coupling caps, so no additional costs there.

Two interstage transformers just can't compare to two mosfets (or 4? one per 300B?)

I agree about the power supply and may try the LED biasing too.

[JPeitzman]

Two interstage transformers just can't compare to two mosfets

In cost or in performance, because if you are talking cost then truth.

Teflon caps, there is a huge part of your cost right there then that you already have, makes a stronger case for the Tubelab or similar design!

I was looking back over the Tubelab SE schematic and the board, then I used my brain a little bit. The very easiest way to do what you want while using a single board would be to hard wire a second socket to the board (1 board 4 sockets), change the output transformer, and diddle with the bias a little bit. The Tubelab uses fixed bias so you wouldn't have to change any parts, just play with the trim pot that adjusts it. The biggest worries I would have here would be power supply ability and drive. I think drive ability would be OK, George would be a better judge of this. You may be taxing the PSU section a bit by running 4 300B's plus drivers off of one 5AR4.

I think the easiest way to do this would be to build it point to point and use veroboard to build your CCS's and FET followers. Unfortunately it doesn't send any financial support George's way and he is a great guy; I feel every penny he makes from the SE's and Simple SE's is more than well deserved. However it allows for a bit more design flexibility.

Some tubes like LED's on the cathode and some don't, word on the street (at least the street I live on) is that the 5842 is one that plays nice with LED's. I have been using them (LED's) recently as I play around with a CCS loaded 264B as a line amp, the jury is still out on that one.

I didn't mean to jump you so quickly about slicing and dicing on the Tubelab board (though I still don't know that I can recommend it). It just isn't always a good idea to take a board designed for one thing and rig it up to do another. Though after looking back, aside from a few issues (the ones mentioned), it doesn't seem like it would be all that hard.

Cheers

James

PS. George or anyone else... if the FET follower is used does it benefit at all from a CCS load verses a large wattage resistor like the Power Drive has? Not to hijack this thread, just interested as Mazuki may want to know too if he is using it.

[Mazuki]

You may be taxing the PSU section a bit by running 4 300B's plus drivers off of one 5AR4.

I don't think the 5AR4 will cut it, so I was planning to use SS rectification with either a standby switch setup or a cheap delay kit off eBay.

Unfortunately it doesn't send any financial support George's way and he is a great guy; I feel every penny he makes from the SE's and Simple SE's is more than well deserved.

I'll most likely buy the board and then decide to use it or go with point to point. I hope I'm not offending George here by modifying his design for more power.

Some tubes like LED's on the cathode and some don't, word on the street (at least the street I live on) is that the 5842 is one that plays nice with LED's. I have been using them (LED's) recently as I play around with a CCS loaded 264B as a line amp, the jury is still out on that one.

Either way, it wouldn't be hard to switch between LEDs and resistors and see what sound I prefer

[JPeitzman]

...standby switch setup or a cheap delay kit off eBay

I would go with the "human powered filament delay" AKA a switch, this is what AudioTropic and I am sure others use. It is cheap, easy, and there are no worries about failure unless you have chubby fingers and you put the B+ and filament switches to close together

Cheers

James

edit: Just out of interest, why parallel, what are you driving?