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GM70 Triode Stereo Oddwatt SRPP / SIPP
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Author:  Soundbrigade [ 22 May 2010, 08:00 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

6N8S aka 6SN7 has a rated Vanode-max 330V.

Author:  freetradeinn [ 22 May 2010, 13:44 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

I use 6H8C as driver in my MA 845 and it has no problems.
Shite as an input, so I use 6sn7 for that.

FTI

Author:  azazello [ 22 May 2010, 15:39 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

freetradeinn wrote:
I use 6H8C as driver in my MA 845 and it has no problems.
Shite as an input, so I use 6sn7 for that.

What kind of driver mode do You use ?

Author:  sampleaccurate [ 22 May 2010, 18:14 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

azazello wrote:
Output signal from 6E5P must be = -U bias. in Your case -100 - -120v !
Please, see Magnus schematic ....+U anode-cathode 6E5P is 140 +_ 5V !!! And Ubias is maybe -100- -120 v !!
Maybe, sorry, something You are wrong.....The schematic is right! Please, think again!
I think that 6E5P is right tube to drive GM70 in 2 stages ampl.!

The bias on the grid will be -120 volts!!! It will be capacitively coupled so no problem there.

Looking at the schematic from Magnus again it shows B+ as 300-440 volts on the 6E5P with only a 1k - 5k resistor to drop the voltage. That being the case it might work.

I don't understand everything you wrote azazello but I think I get the main point. I'm still thinking about reactive loads where there's not a big DC drop across the load. That's where I went wrong. According to magnus the anode can be pushed to 320 volts.

Author:  azazello [ 23 May 2010, 09:20 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

Hi,
Ra is not only droper resistor, it is Ra = Ri 6E5P x /3 - 6/.
Ri is aprox. 2 kohm and Ra must be 6-8 kohm!! But not 1 kohm and more!!
If current is 20-25 ma, U on Ra will be 120-150 v.
And Ua 6E5P will be 150 v and more, that is enough
to drive GM70 with -100-120 v bias.
Magnus's schematic is really very good!
Go ahead!
/be carreful with 1000 v!!!/.

Author:  sampleaccurate [ 23 May 2010, 19:47 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

azazello wrote:
Ra is not only droper resistor, it is Ra = Ri 6E5P x /3 - 6/.
Ri is aprox. 2 kohm and Ra must be 6-8 kohm!! But not 1 kohm and more!!
If current is 20-25 ma, U on Ra will be 120-150 v.
And Ua 6E5P will be 150 v and more, that is enough
to drive GM70 with -100-120 v bias.
Magnus's schematic is really very good!
Go ahead!
/be carreful with 1000 v!!!/.

Makes sense. Thank you. I may end up using a pentode driver on one GM70 pair, and on the other a SRPP stage using a dual triode, and see what works the best.

I must say I'm almost tempted to just build the SE circuit. My 300B sounds better than I thought an amp could, I LOVE the way it distorts (or doesn't distort), and the power output is higher than I thought. I'll stick with a push pull GM70 for now, but the only hard part about changing from PP to SE is buying new transformers. I could use the old ones in another amp.

I'll model the 6E5P portion of the SE GM70 circuit and see how it behaves as a pentode.

The only issue I haven't figured out is what to do if the AC voltage on the cathodes in the SIPP stage exceeds 55 volts (or more realistically 50 volts). If it does I need to redesign the CCS. With KT88s it's low, maybe 20 volts as I recall (don't quote me on that). With a much higher voltage tube like the GM70 it could exceed 50 volts - I just don't know.

Author:  sampleaccurate [ 27 May 2010, 01:42 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

The latest on this is it's complicated. Finding an inexpensive driver for the GM70 is tough. I've tried a lot of different ideas.

My latest plan right now is a 6SN7. And it can't be an SRPP stage. Ben was kind enough to point out I was exceeding heater - cathode voltages in a previous SRPP design. It's the heater-cathode voltage that limits the voltage output of an SRPP stage. It simulates well on PSPICE, but you don't realize you're burning up your tube. The 6SN7 can handle relatively high plate and heater to cathode voltages, and is one of few tubes I can find that can produce a clean 240 volt peak to peak voltage output. The only drawback of the design is it requires an additional cap to allow the voltage gain neccessary - it can't be direct coupled and not exceed heater - cathode voltages on one triode or the other or achieve the necessary output swing either direct coupled or in SRPP configuration. But, it appears it can be done in two capacitively coupled gain stages.

I found some high voltage transistors that will extend the voltage capability of the LM317HV CCS to 450 volts, up from about 55 volts with the LM317HV alone.

The layout has been determined. It's a tight fit, but it's doable as a stereo amp in one chassis. It will be PP (SIPP).

So essentially the design is complete and all the parts are on the way.

Author:  Bennybuis [ 27 May 2010, 02:59 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

Hi Stephen,

good job so far, I am very curious about how it will sound. I haven't had the liberty to listen to a triode pp amp so far, tempted to build one just because of that. I prefer a pp amp any day over se, we had a lot of discussions about that. That is just my opinion based on my very own experience. It is true, a pp amp is much more tedious to build and to develop a world class amp you need some tools like a good signal generator and o-scope. Not much to archive with intuition, I tell you. Lots of folks prefer se amps for the simple reason, those are much easier to build and much more forgiving, but when it comes to reproducing music, a well build pp amp will outperform any se amp any day. (IMHO)

Author:  freetradeinn [ 27 May 2010, 08:33 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

freetradeinn wrote:
I use 6H8C as driver in my MA 845 and it has no problems.
Shite as an input, so I use 6sn7 for that.

azazello wrote:
What kind of driver mode do You use ?

Hi sorry for the delay (diagrams and final pics are finished tonight!!!!!!!!!!!!!!)

The input and driver stage are Common cathode (6sn7) and Cathode follower (6H8C) respectively.

FTI

Author:  sampleaccurate [ 27 May 2010, 08:41 ]
Post subject:  Re: GM70 Triode Stereo Oddwatt SRPP / SIPP

Bennybuis wrote:
good job so far, I am very curious about how it will sound. I haven't had the liberty to listen to a triode pp amp so far, tempted to build one just because of that. I prefer a pp amp any day over se, we had a lot of discussions about that. That is just my opinion based on my very own experience. It is true, a pp amp is much more tedious to build and to develop a world class amp you need some tools like a good signal generator and o-scope. Not much to archive with intuition, I tell you. Lots of folks prefer se amps for the simple reason, those are much easier to build and much more forgiving, but when it comes to reproducing music, a well build pp amp will outperform any se amp any day. (IMHO)

My guess is I'll agree with that assessment after I finish my KT88 oddwatt (it's sooooo close).

I typically don't like distortion. It would be fair to say I hate distortion where it's not wanted, especially hard clipping. But the SET amp is "strange". The distortion that is produced at low drive levels is inaudible to my ears, even though it's probably well over 1% (I'm guessing based on what I've read - I didn't measure it). When you "crank it up" it's difficult to determine how loud you can turn it. You can basically (speakers allowing) turn it up as loud as you want to depending on how much distortion you can tolerate. The thing is, the distortion is actually pleasant on some sources. I now know the meaning of the term "euphonic distortion". A little bit of 2nd order harmonic distortion can make strings and voice "come alive". The amp is actually NOT reproducing the source faithfully, it's introducing low order even harmonic distortion. The "artificially created octave" which is essentially what it's generating adds something to the sound.

This is great - sometimes. I certainly don't want to always be limited by power and distortion. If I had to choose one amplifier to own, it would be a higher power amp with very low distortion, whatever that amp may be. It would not be a SET. However, this amp has its place, it sounds awesome IMHO and behaves very differently than any SS amp or any amp for that matter I've ever owned. That's why I like it so much. It "chimes". Having played tube guitar amps for decades I can hear the hint of distortion, but it's perceived as an increase in clarity and richness rather than distortion as long as it's not driven too hard. This effect is especially apparent on strings and voice. It's basically good sounding distortion, if there is such a thing in the world of hi-fi.

I want my PP amp to have less distortion and more power, and it will. What that translates to in terms of sound I can't wait to find out. I'm expecting a "tighter" sound than the SET, more comparable to a good SS amp (but better of course), and I don't anticipate that the 3rd order distortion component (and higher order odd harmonics) will be pleasing to the ear. I don't anticipate purposefully driving the amp into distortion like I do with the SET.

It's a long holiday weekend here in the US so maybe that oddwatt will be finished and fired up in a few days.

Thanks for the help.

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