Octal Mullard 5-20 stereo with KT88
- Thread starter Marsman
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At least a few AK'ers have made 5-20-style amps -- @kward and myself among them. I don't think Kevin has used a small signal pentode in any of this, and I know I haven't (opting for a dual triode instead, because stereo and also because I don't usually need all the gain the pentode provides). Kevin and I are also fans of using the 6SN7 in the phase inverter position, and I usually put a constant current sink in the tail of mine.
Dandy
Addicted Member
Hi Marsman, welcome to AK. I'm not able to comment on the design changes, but I note that the EF37A and the EF86 have quite similar specs, but why substitute the EF37A for the EF86? Maybe investigate Jason's dual triode approach instead.
Sounds like an octal substitute for the ECC83 would work, Any reason for using 6SN7 rather than the equivalent 6SL7 for phase inversion?
Just more questions! Please keep us posted on your build.
Sounds like an octal substitute for the ECC83 would work, Any reason for using 6SN7 rather than the equivalent 6SL7 for phase inversion?
Just more questions! Please keep us posted on your build.
Last edited:
s-petersen
Scott
Maybe sub Kt-88's in this? Pilot 901
A 6SL7 for the voltage amp state + 6SN7 for phase inverter should work pretty well, but you might need more voltage gain to be able to drive KT88s to full power after gNFB with reasonable input sensitivity. You could improve the gain of the 6SL7 there by using a constant current source for the load.
I like 6SN7s because in the 5-20 topology with a high-mu triode in front, it's easy to run at the right operating point for direct-coupling.
I've built basically the amp in question here, except with a 12AX7 as the input tube (in fact, a version of the "Hot Rod" Eico ST70 as published on the Tronola website, modded to use KT88s instead of paralleled 7591s). Everything else was basically as described in the OP- a 6SN7 LTP inverter, and KT88/ 6550 output tubes.
I wound up with an input sensitivity of 1.4v for full power- about what a Dynaco ST70 requires. Worked fine.
However- with the 6SL7- gain of 70 instead of 100 for the 12AX7- that means that with the same amount of feedback, it would take about 2.0v to do full power. Not terrible- but more than many "common" amps...
Now- and here's the kicker- that WAS with a HUGE amount of feedback- like something on the order of 27dB! If you reduced that to a more "normal' value, you could probably do fine with the 6SL7. I'd think you could probably do 20dB feedback, and wind up with about 1v input sensitivity...
Regards,
Gordon.
Dandy
Addicted Member
Why shouldn't the OP add a weblink? Nice to see other AK members take a different attitude, and offer some ideas.
s-petersen
Scott
I think the complaint was that the op owns a business, building amplifiers and the sig points to it, and they aren't a sponsor.
Marsman
ampbuilder
Thanks for the replies. Dandy, I have a box full of 6J7 and EF37's that are waiting to be used.
Point taken s-petersen. We'll subscribe a.s.a.p.
I think using the EF37/6SN7 to make a 5-20 makes considerable sense. Both tubes conform to the circuit requirements. I built two 5-20 type circuits last year. One was 12BY7 into 5687 and the other EC8010 into 6F8G. Experimenting is fun and while not every circuit works as well as one might wish, let's hope they are learning experiences.
primosounds
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Hi All, this is my first thread on AK
I am constructing an amplifier that is inspired by the Mullard 5-20 amplifier. Changes to the original design are: EF37 input pentode, 6SN7 phase inverter/driver and KT88 output tubes.
Has anybody attempted something like this?
The EICO HF60 is a similar Mullard circuit. In the original design an EF86 is direct coupled to the 6sn7 phase splitter which is cap coupled to the EL34 output tubes. This is a fixed bias amp and thus is capable of around 50 watts / channel from the Acrosound OPT. One of the best mods is to adapt the circuit for kt88 output tubes which the transformers are actually better suited.
Marsman
ampbuilder
Thanks for that comment, Pio1980. That Marantz 8 is a gem. I am going to look at the schematic today.
That is why I chose a pentode frontend, so I have the option of triode mode for less gain.
Marsman
ampbuilder
primosound, the EICO HF60 is actually the inspiration for this build. There is a thread on the web ( http://tronola.com/html/hf-60_with_6550s.html ) that I found inspiring. So one could also call this build an "autobias HF60 with KT88"
primosounds
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Ah yes, i am very familiar with that article. I wonder why you would rather make your circuit cathode biased instead of fixed bias on the grid?
Dandy
Addicted Member
Will you aim to replicate the Eico HF60 output transformers?
primosounds
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I am not sure that cathode bias is more reliable but it is less "problematic" especially for a tube newbie. The only thing i don't like about cathode bias for a kt88 is that you will be using 4 x 7 watt cathode bias resistors which will dissipate a lot of heat. Not only is that wasted power but also you need to account for locating the resistors in a location to properly vent that heat. Another thing is that the electrolytic caps for bypassing the resistors will have a significant effect on the sound quality.