807 ST70?

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807 ST70?

Postby Shannon Parks » Fri Oct 20, 2006 6:00 am

Since everyone seems to be jump starting new tube projects this fall, let me start up a thread about Thermion's new madness. Unfortunately, the Ikezilla will have to wait a little while longer. Hey, eight KT88s alone add up, let alone all the hardware, right? :o

During our last couple visits to Peoria Superfest, Thermion has grown to love those 1625 and 807 transmitter tubes. After our last trip, the Doctor said something like the 807s were rated to do about 55W in a typical 450V type amp. I called BS immediately. But the Doctor is a walking tube resource without parallel, and he was indeed correct. So Thermion has cobbled together the following: ST70 chassis with PA060, Hammond 1650P pair, a diytube driver board (mods?), some 5-pin sockets from last year's hamfest, and a bunch of cool brown base RCAs. Not sure what rectifier he will use, but I hope we can do tests with UF5408s, too. Should go together pretty quickly.
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Postby EWBrown » Fri Oct 20, 2006 8:08 am

The 6BG6G is somewhat similar in character to the 807 and the 1625, just the pin basing arrangements and/or filament voltages are different. Nice thing with 807s, they are available as NOS plentifully and cheaply, and there are current-day Russian and Chinese 807 equivalents, both of which are very good. 1625s are an often-overlooked but very nice tube. Yellow_Light_Colorz_PDT_11

The olde-timey radio ops and hams often refer to bottles of beer as "807s" as well... Yellow_Light_Colorz_PDT_06

55W is about right for a 400VDC circuit, and they can be pushed up to about 80W CCS and 120W ICAS (not suitable for real audio usage).

807: http://www.mif.pg.gda.pl/homepages/fran ... /GL807.pdf

1625: http://www.mif.pg.gda.pl/homepages/fran ... 1/1625.pdf

6BG6G: http://www.mif.pg.gda.pl/homepages/fran ... /6BG6G.pdf


/ed B in NH
Last edited by EWBrown on Tue Mar 11, 2008 9:59 am, edited 1 time in total.
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Postby mesherm » Fri Oct 20, 2006 9:53 am

I definitely like the sound of 6GB6Gs but they can be a bit tricky to bias. I have had to boost available bias voltage in some cases to -80 volts. I had some NOS RCA 6BG6GA tubes that wouldn't even bias at THAT voltage.
I have had good luck with some newer NOS 6BG6GAs although the ST bottle 6BG6Gs look the best.
Nice NOS Phillips 6BG6GAs can be had here:
http://www.vacuumtubes.com/6BG6.html

I am in the process of modifying one of my two 6BG6 amps with rotary switchs so I can switch between 3 or 4 different tube socket pinouts. Sort of a universal PP amp. I am also adding switchable 6/12 volt filament supply. This way I can run 6L6, 6AV5, 6BG6G, and one as yet undetermined pinout along with any 12 volt cousins. I had planned to install sockets or jacks to attach plate cap wires for them tubes whats needs them.
I will report further when it is complete.

BTW, Good luck trying to get the right ceramic plate caps. I ordered a set of 4 from two popular tube supply houses and got 5/16" caps instead of 3/8" in both cases. I didn't find this out until months after I received them. I gave up after that and made my own.
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Re: Biasing

Postby Shannon Parks » Fri Oct 20, 2006 11:57 am

Hmmm - I wonder if these high bias tubes might work better in a mixed bias environment. Say a 100 ohm cathode resistor, bypassed, with a fixed bias supply doing the rest.
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Postby EWBrown » Fri Oct 20, 2006 1:16 pm

I have one idea for a "mixed bias" scheme for an impending PPP EL84 amp, use a 300-400 ohms cathode resistor for each EL84, and then "trim" with a 10K pot, the ground end of each grid resistor with a +5 to -5 VDC bias supply (a good use for that otherwise unused 5V filament winding) just a simple 2 diode voltage doubler, but referenced to ground through the "cold" end of the 5VAC winding.

That should be enough adjustment voltage "slack" for all those "oddball" unmatched 6P14Ps which are closer to the 10V or 16V K to G1 voltage ends of the 35 mA bias spectrum, instead of being fat and happy around 12-14 VDC..

Just another wild thought from an underutilized mind... Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_11 Yellow_Light_Colorz_PDT_06
Last edited by EWBrown on Mon Dec 03, 2007 7:41 am, edited 1 time in total.
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Postby SteveH » Thu Nov 22, 2007 12:58 pm

Shannon,
So what became of the 807 amp ? I was going to try out the MK3 6550 route; but alas my 'source' for the chassis and trannies fell through - he did give me a very crusty looking ST70 though - seems like 807s would be fun to try....

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Postby Geek » Thu Nov 22, 2007 4:28 pm

That would be cool to find out how it went.

Basically, 807 == 6L6GC for RF and in a different bottle. Sounds about the same too.

My buds and I have swapped them and the sockets for 6L6's and didn't even need to rebias beyond the norm for changing tubes Yellow_Light_Colorz_PDT_11

Haven't tried a 6BG6 yet...

Cheers!
-= Gregg =-
Fine wine comes in glass bottles, not plastic sacks. Therefore the finer electrons are also found in glass bottles.
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Postby SteveH » Thu Nov 22, 2007 5:47 pm

Gregg,
Ya - I know that basically any 6L6 circuit can take 807's no problem; just need to swap the sockets.
I wonder how much they could be cranked up ? I imagine you'd want to stay under 40w with the stock trannies; but edcor makes some nice 60w ones though...Then it would just be a matter of using Triode's updated power tranny perhaps ?
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Postby Geek » Thu Nov 22, 2007 10:43 pm

Depends on the quality of the drive. 40W would be really clean methinks. Add new Edcor iron and you can push them somewhat further and with better bass. But 40W to 60W isn't that significant. 40W to 80W would be just discernable at 3dB increase.

Cheers!
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Postby burnedfingers » Mon Mar 10, 2008 8:42 pm

So you could use either 6L6's or 807's in the St70? Any changes needed besides different sockets to use the 807?
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Postby kheper » Tue Mar 11, 2008 12:38 am

Although intuitively, just dropping in some
sockets and rigging up the anode lines with
anode caps appears to work - somewhat
adequately, an 807 is not an EL-34, 6L6,
or a 6L6GC, in a cool looking disguise.

The 807's screen grids are rated at 300V.
Hooking up 450 to them, apparently, does
not make fireworks, but over-volting the
screens will make them very prone to
'self-oscillation' and thermal runway.

Putting a resistor in the range of 10k on the
screens to chop down the voltage to around
300V is absurd. There will be heat a-plenty,
a loss of regulation, a loss of power and
increased distortion.

An 807 is not the perfect candidate for
'normal UL operation', but it can be done
with a few provisos. An output transformer
with 10k primary impedance and a %50
screen tap is the best insurance.

The 807, on paper, has the same plate
dissipation as the EL34, but it in the real
world, it is closer to 22W. For an AB1 PP
amp, the tube should run at a Pda of about
2/3 rated total tube Pd.

So, 2/3 x 25(on paper) = 16.5 W

I am planning on running the B+ at around
350V. I have a 280Vac @ 600ma plate
transformer. With a full-wave bridge and a
big choke, after loading, it should get to
around 350V or lower. At 350V, there
should be some sweet class A watts.

Ea = 350V
Ia = 44ma
Eg2 = 350V
Ig2 = 8ma

Pda = 350V x 44ma = 15.4

Pdg2 = 350V x 8ma = 2.8

Total tube dissipation is 18.2

18.2 is a little high, but if the 4ohm tap is
used for 8ohm speakers, the tubes will not
as likely lead to thermal runaway.

Hi-fi amps are at their best with a low Ea
a low Eg2, and a high idle Ia. With the 807,
a low % of the max Eg2 is not really
achievable with normal output transformers.
In addition, 807s do not like to be biased
higher than 30ma.

Below is the most comprehensive source of
data for the 807:

http://www.retrovox.com.au/STC807.pdf
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Postby Shannon Parks » Tue Mar 11, 2008 6:18 am

Thermion has had his 807 ST70 up a while now. Basically ST70 voltages with Hammond 6.6K 1650P outputs hooked up UL. I think he gets around 22W at 1%THD. No stability issues that I've heard of, and I think he is using the stock diytube board tuning. I would like to see it with some 10kHz square waves and different loads. I keep asking him to do a write-up - hopefully soon!
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Re: 807 ST70?

Postby Thermion » Tue Jul 15, 2008 10:16 pm

separks wrote:Thermion has grown to love those 1625 and 807 transmitter tubes. After our last trip, the Doctor said something like the 807s were rated to do about 55W in a typical 450V type amp. I called BS immediately. But the Doctor is a walking tube resource without parallel, and he was indeed correct. So Thermion has cobbled together the following: ST70 chassis with PA060, Hammond 1650P pair, a diytube driver board (mods?), some 5-pin sockets from last year's hamfest, and a bunch of cool brown base RCAs. Not sure what rectifier he will use, but I hope we can do tests with UF5408s, too. Should go together pretty quickly.


Indeed, this amp went together pretty quickly. So far, it is the first amp I have ever built with no problems. And it was cobbled together with the parts Shannon mentions. It powered up the first time. Never ocillated. No votage run-aways. I was having trouble with another ST70 and my MKIII's, so it has been in my main system driving a pair of Apogee Centaurs for the last year+. The amp is very musical sounding, and didn't mind being taxed a little on some music.

The power supply on this amp is essentially stock ST70. Even the quad cap is original. The board was cut-and-jumpered to allow 6CG7 tubes. I used a GZ37 rectifier tube to tame the voltages on the 807 output tubes. With 117 vac on the line, I got a B+ of 361 v, 360 v on th screens, and 356 v on the plates. The outputs are run in UL mode.

Tube compliment is all RCA, Four nearly matched used 807 tubes from various sources, two 6CG7 tubes, and one 12Ax7. Circuit parameters were basically stock DIYTUBE values except for the following changes:

R20-R22 (LTP plate resitors): 47.5K
R23-26: 90.9K
C7-C10: 0.47 uf

Feedback values for R9, R10, C3, C4, R13, R14, C5, and C6, are on your own. I have not optimized these values for this amp. Although, it has been stable on a pretty complex load. I'll post these values as they get tweaked.

With the GZ37 rectifier, I get a 279 v B+ on the LTP with a voltage drop of 125v for a current of 2.6 ma through each section of the 6CG7. The 12ax7 GC had a B+ of 235 with a voltage drop of 98 for a current of 1 ma. The bias on the 807's is 56 ma.

With this arrangement, I got 24 db of gain with 13 db of negative feedback. The 1% at 1Khz distortion power was 22 watts.

Anyway, I am getting ready to tweak this amp against my better judgement because it has been flawless so far. This is my first 807 amp and I really love these tubes. They are so musical. And they are cheap and available. Over two years I collected about 30 807's and 30 1625's. Several well matched pairs and a few matched quads. These are 40's and 50's vintage RCA's, Ken-Rad, Nation Union, etc. And I usually find these for a few bucks apiece.

I am hoping to do some sweeps in the near future and tweak the the feedback values.

Regards,
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Postby Shannon Parks » Wed Jul 16, 2008 6:25 am

After auditioning the MKIII's w/ Poseidon's this past weekend, I'm thinking a single, HV axial cap in parallel with the second section of the can cap would provide the same effect. Maybe two of these Nichicon 100uF, 350V in series, adding 50uF.
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Re: 807 ST70?

Postby kheper » Fri Jul 18, 2008 4:15 pm

Thermion wrote:
separks wrote:Thermion has grown to love those 1625 and 807 transmitter tubes. After our last trip, the Doctor said something like the 807s were rated to do about 55W in a typical 450V type amp. I called BS immediately. But the Doctor is a walking tube resource without parallel, and he was indeed correct. So Thermion has cobbled together the following: ST70 chassis with PA060, Hammond 1650P pair, a diytube driver board (mods?), some 5-pin sockets from last year's hamfest, and a bunch of cool brown base RCAs. Not sure what rectifier he will use, but I hope we can do tests with UF5408s, too. Should go together pretty quickly.


Indeed, this amp went together pretty quickly. So far, it is the first amp I have ever built with no problems. And it was cobbled together with the parts Shannon mentions. It powered up the first time. Never ocillated. No votage run-aways. I was having trouble with another ST70 and my MKIII's, so it has been in my main system driving a pair of Apogee Centaurs for the last year+. The amp is very musical sounding, and didn't mind being taxed a little on some music.

The power supply on this amp is essentially stock ST70. Even the quad cap is original. The board was cut-and-jumpered to allow 6CG7 tubes. I used a GZ37 rectifier tube to tame the voltages on the 807 output tubes. With 117 vac on the line, I got a B+ of 361 v, 360 v on th screens, and 356 v on the plates. The outputs are run in UL mode.

Tube compliment is all RCA, Four nearly matched used 807 tubes from various sources, two 6CG7 tubes, and one 12Ax7. Circuit parameters were basically stock DIYTUBE values except for the following changes:

R20-R22 (LTP plate resitors): 47.5K
R23-26: 90.9K
C7-C10: 0.47 uf

Feedback values for R9, R10, C3, C4, R13, R14, C5, and C6, are on your own. I have not optimized these values for this amp. Although, it has been stable on a pretty complex load. I'll post these values as they get tweaked.

With the GZ37 rectifier, I get a 279 v B+ on the LTP with a voltage drop of 125v for a current of 2.6 ma through each section of the 6CG7. The 12ax7 GC had a B+ of 235 with a voltage drop of 98 for a current of 1 ma. The bias on the 807's is 56 ma.

With this arrangement, I got 24 db of gain with 13 db of negative feedback. The 1% at 1Khz distortion power was 22 watts.

Anyway, I am getting ready to tweak this amp against my better judgement because it has been flawless so far. This is my first 807 amp and I really love these tubes. They are so musical. And they are cheap and available. Over two years I collected about 30 807's and 30 1625's. Several well matched pairs and a few matched quads. These are 40's and 50's vintage RCA's, Ken-Rad, Nation Union, etc. And I usually find these for a few bucks apiece.

I am hoping to do some sweeps in the near future and tweak the the feedback values.

Regards,
JT


You have refreshed my interest in this project. I have a power supply that will yield about 350V loaded in a full wave bridge rectifier configuration. The transformers are PP UL Transcendars. Should be a sweet sounding amp. I hope it is not power shy, however.
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