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Old 30th November 2012, 08:28 PM   #11
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The colour of the base is different mainly because of the time:

Ulyanovsk factory production (old logo), black base:

Click the image to open in full size.

Again Ulyanovsk factory production (new logo), brown and black base:

Click the image to open in full size.

GM70 in parallel is something useless except that case,
when you're using speakers with extremely low sensitivity
(or electric heater instead of speaker).
Tube SE amp (as a conception) also includes high sensitivity full range driver,
mounted (very often) in a BL horn enclosure.
Anyway, 20-25 watts of output power is enough
even for speakers with sensitivity about 82-84dB/W/m.
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Last edited by widowmaker; 30th November 2012 at 08:44 PM.
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Old 30th November 2012, 10:05 PM   #12
kevinkr is offline kevinkr  United States
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I can attest that the >20W I get out a single copper plate GM70 is more than sufficient with my speaker system, and frankly if you need more power than this you might want to consider something other than a GM70 in A1..

You could for example IT couple your triode connected 807 to the GM70 run it fairly hard (say 50mA or so) and you might get far enough into A2 to get 40 - 50W out a single GM70. I'd probably do a 2:1 step down or similar (3:1?) that should give you plenty of grid current capability. Note mine is designed primarily for A1and I have not tried this yet.. (Won't with these amps as I am very pleased with their overall performance)
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Old 1st December 2012, 12:34 PM   #13
dmac23 is offline dmac23  United States
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Well in most situations I would not be concerned about wattage. I used to own some horn speakers and remember being happy with a 6 watt SET. Unfortunately my goal with this project is to power some large infinity's... Before you think I am crazy, I have tried all kinds of power amps on these speakers. To my amazement, they are very versatile when it comes to the variety of amplifiers that make them sing. They sound excellent with a 40 watt el34 amp and I have found that 50-75 watts on these is more than I really need. So my thought is if I could get that out of a SET it would be pretty cool.

...ok and I just may want to build a GM-70

My guess is that A2 will have higher distortion than A1 operation, correct? Would the B+ have to go up also?

Thank you guys for the guidance, I really appreciate it!!

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Old 1st December 2012, 03:09 PM   #14
hooman is offline hooman  Iran
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Quote:
Originally Posted by dmac23 View Post
Thanks everyone for the advice, I will look at some old radio books and get a clearer image on the HV supplies.

I looked up the russian 807 on ebay and could not believe the low price on these! How does the sound compare to Sylvania or RCA?

Also no one commented on the thought of running them in parallel to get more power, are there any complications that may occur by doing this?
hi
this tubes make your amp very dark sounding.
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Old 1st December 2012, 05:48 PM   #15
dmac23 is offline dmac23  United States
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Quote:
Originally Posted by hooman View Post
hi
this tubes make your amp very dark sounding.
Are you referring to the Russian 807? or the 807 in general?
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Old 1st December 2012, 08:48 PM   #16
glennb is offline glennb  Australia
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Quote:
Originally Posted by dmac23 View Post
... I am thinking of using a 6sn7 type as an input tube followed by an 807 to drive the gm-70ís....
If you want to consider avoiding the use of a tube with a Top Cap (807), the EL34 in triode mode can swing a lot of voltage with relatively low distortion. 801A is another possibility if you want to try DHT.

For maximum power output on GM70 (and 845) the anode voltage should be as high as possible and bias it as hot as possible, to run close to maximum anode dissipation.
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Old 1st December 2012, 08:54 PM   #17
Gluca is offline Gluca  Italy
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I'd take 801A any day.
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Old 2nd December 2012, 01:12 AM   #18
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by dmac23 View Post
<snip>
...ok and I just may want to build a GM-70

My guess is that A2 will have higher distortion than A1 operation, correct? Would the B+ have to go up also?

Thank you guys for the guidance, I really appreciate it!!
Same B+ and since it is class A the average power consumption does not change. (The instantaneous power consumption will increase and low supply impedance is important.)

Results will depend almost entirely on how linearly the driver stage can provide the required grid current. The GM70 curves in the grid current region still look very good.
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Old 2nd December 2012, 04:13 AM   #19
kheper is offline kheper  United States
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Patrick Turner uses 3 el-84s in triode mode to drive his PSE 845 mono-block. The el-84s produce "164Vrms of signal output". He said that he was "reduced to tears", after listening to this amp.

monobloc845se55
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Old 2nd December 2012, 09:17 AM   #20
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Default class A1 or A2?

Hi

Do you intend to drive the GM70 in A2 or A1?

You can have a look how I managed this on my TB3/1000 150W SE amp. I am using an EL34 simply because I like this tube very much.

ward's 150W SE amp blog part 3

By the way, we have a GM70 output transformer in our range. // version can be made with very good spec that are comparable to the ones of the single GM70.

http://www.monolithmagnetics.com/ima...sheet_S-15.pdf

We also produce inductors , low capacitance versions if needed on a custom basis. The low capacitance versions (each layer isolated individually) will accept very high voltages with no problem.

You can indeed put them in the zero line as Kevin indictaed.

I wish you succes with your project.

grz, //WDC
www.monolithmagnetics.com
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