I want to build an amp using what I've got around. I've got two 6l6s, 5u4, several 6SH7s and an 6SC7. My PT is like 490-0-490 and beefy. It came in an old phono type amp with the two 6L6s.
So I haven't used octal preamp tubes before and have never used pentode preamp tubes either. I'm thinking this basic design: Two 6SC7 (dual triode) gain stages into one 6SH7 (single pentode) gain stage into a 6SH7 cathodyne. I wasn't sure you could make a cathodyne with a pentode because i've never seen it and google didn't help so i modeled it in SPICE with an EF86 and it seems to work fine. I figure with the pentode stage before the cathodyne there should be plenty of swing to drive the two 6L6 in push pull.
But then I was thinking well what if I just did two pentode gain stages using the two 6SH7s into a 6SC7 long tail pair? That would use what I got and let me see the pentode flavor preamp much more that the other design would. My only hesitation is I have heard 6SH7 can be rather noisy/microphonic so you wouldn't want to use them for the first stage of an amp.
What do you all think? Any suggestions? Thanks guys
Possible octal pentode/triode designs using what i have?
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David Root
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- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
Re: Possible octal pentode/triode designs using what i have?
I have run octal double triodes, especially the 6SL7, in high gain circuits. They sound really good, BUT you MUST use 6V DC on the heaters because these old tubes were not designed to be used in multistage high gain circuits. The filaments are also not like 12A*7 types, which are designed to be used in a humbucking connection. The octals do not have the humbucking wiring option.
If you try it with standard 6V AC on the heaters you will be buried in hummmmmmm!
The PT you reference is way too hot for most modern (and NOS for that matter) 6L6GCs, you should get about 600V plate voltage even with the 5U4G. NOS 7581A or 7027A can handle it most likely, but VERY expensive. I have heard Winged C 6L6s can handle that but never tried it. You would need to bias about 25 mA cathode current per tube, maybe less, which helps. What are the 6L6s you got with it?
I think it's doable but you don't need all the stages you are thinking about using, and be careful with the cathodyne PI design, if you overload it it can make seriously nasty tones!
On the cathodyne, if you have access to a copy of Merlin Blencowe's Designing Preamps fot Tube Guiar and Bass he has a good chapter on making the cathodyne work right.
Go for it!
If you try it with standard 6V AC on the heaters you will be buried in hummmmmmm!
The PT you reference is way too hot for most modern (and NOS for that matter) 6L6GCs, you should get about 600V plate voltage even with the 5U4G. NOS 7581A or 7027A can handle it most likely, but VERY expensive. I have heard Winged C 6L6s can handle that but never tried it. You would need to bias about 25 mA cathode current per tube, maybe less, which helps. What are the 6L6s you got with it?
I think it's doable but you don't need all the stages you are thinking about using, and be careful with the cathodyne PI design, if you overload it it can make seriously nasty tones!
On the cathodyne, if you have access to a copy of Merlin Blencowe's Designing Preamps fot Tube Guiar and Bass he has a good chapter on making the cathodyne work right.
Go for it!
Re: Possible octal pentode/triode designs using what i have?
Hmm. I had planned on 6v AC for the heaters. So I should use DC then? Just preamp or whole shabang?
The 6L6 that came with it are actually Mesa branded 6l6. Thanks ebay.
I knocked down the screen voltage to around 400V with a 1K screen resistor and a B+ dropping resistor around 120K. That gives around 500V B+ which then gets cut down through the screen resistor. I'm worried a dropping resistor that big will give way too much sag so I may try EL34 instead..
The 6L6 that came with it are actually Mesa branded 6l6. Thanks ebay.
I knocked down the screen voltage to around 400V with a 1K screen resistor and a B+ dropping resistor around 120K. That gives around 500V B+ which then gets cut down through the screen resistor. I'm worried a dropping resistor that big will give way too much sag so I may try EL34 instead..
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David Root
- Posts: 3540
- Joined: Fri Aug 04, 2006 3:00 pm
- Location: Chilliwack BC
Re: Possible octal pentode/triode designs using what i have?
DC heaters on every octal tube except the power tubes (assuming push-pull output).
MESA 6L6s came in two flavors, STR-387 and STR-415. 387 is a fairly strong 6L6GC. 415 is a 7581A, which is rated for 500Vp and 35W Pd. Screen is rated for 450V so you can probably do 500 Vp and Vs OK.
You are right about the 120K B+ dropping resistor, don't do it. If you prefer a 6L6 type tone try a 6CA7 instead of EL34. Or use 6550/KT88.
MESA 6L6s came in two flavors, STR-387 and STR-415. 387 is a fairly strong 6L6GC. 415 is a 7581A, which is rated for 500Vp and 35W Pd. Screen is rated for 450V so you can probably do 500 Vp and Vs OK.
You are right about the 120K B+ dropping resistor, don't do it. If you prefer a 6L6 type tone try a 6CA7 instead of EL34. Or use 6550/KT88.