Last edited by PeterCapo on Thu Dec 03, 2020 10:07 pm; edited 1 time in total
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jrubin
Bob Latino
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GP49
mazeeff
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11 posters
Barn find ST70
Guest- Guest
- Post n°26
Re: Barn find ST70
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Last edited by PeterCapo on Thu Dec 03, 2020 10:07 pm; edited 1 time in total
GP49- Posts : 792
Join date : 2009-04-30
Location : East of the sun and west of the moon
- Post n°27
Re: Barn find ST70
Reads OK to me, Peter.
In #2, the electrolytic capacitors in the bias section appear to be "flipped around" because the bias supply is negative. This HAS tripped up even knowledgeable builders since almost all the time in the heyday of tubes, we dealt with positive supplies.
In #8, the return path is through the EL34s, then through the common cathode resistor to ground. The DC current through the tubes and then the resistor raises the high end of the resistor above ground, according to Ohm's Law, and that is what we measure when setting bias...we are not actually measuring the bias, which is the negative voltage on the grids of the EL34s. As the bias varies, the cathode current and the voltage on the cathode resistor varies, a numerically higher negative bias on the grids causing lower quiescent current and a lower voltage on the cathode resistor. Consider also that any signal through the amplifier also flows through the transformer secondary, the EL34s, and the cathode resistor, so that you can't set the bias while the amp is passing a signal.
In #2, the electrolytic capacitors in the bias section appear to be "flipped around" because the bias supply is negative. This HAS tripped up even knowledgeable builders since almost all the time in the heyday of tubes, we dealt with positive supplies.
In #8, the return path is through the EL34s, then through the common cathode resistor to ground. The DC current through the tubes and then the resistor raises the high end of the resistor above ground, according to Ohm's Law, and that is what we measure when setting bias...we are not actually measuring the bias, which is the negative voltage on the grids of the EL34s. As the bias varies, the cathode current and the voltage on the cathode resistor varies, a numerically higher negative bias on the grids causing lower quiescent current and a lower voltage on the cathode resistor. Consider also that any signal through the amplifier also flows through the transformer secondary, the EL34s, and the cathode resistor, so that you can't set the bias while the amp is passing a signal.
Last edited by GP49 on Tue Oct 06, 2015 6:57 pm; edited 1 time in total
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°28
Re: Barn find ST70
I removed my dumb question.....
Incidentally pins 4 and 6 on the GZ34 should be read with respect to ground and not each other. reading across 4 and 6 doubles the voltage reading where 4 and 6 to ground shows 360vac
however, if someone could beasure 4-6 with the rectifier removed for resistance (with the power off) id like to know if they match my reading of 61ohms
Jordan
Incidentally pins 4 and 6 on the GZ34 should be read with respect to ground and not each other. reading across 4 and 6 doubles the voltage reading where 4 and 6 to ground shows 360vac
however, if someone could beasure 4-6 with the rectifier removed for resistance (with the power off) id like to know if they match my reading of 61ohms
Jordan
Last edited by jrubin on Tue Oct 06, 2015 6:42 pm; edited 1 time in total
Guest- Guest
- Post n°29
Re: Barn find ST70
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Last edited by PeterCapo on Thu Dec 03, 2020 10:07 pm; edited 1 time in total
Guest- Guest
- Post n°30
Re: Barn find ST70
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Last edited by PeterCapo on Thu Dec 03, 2020 10:08 pm; edited 1 time in total
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°31
Re: Barn find ST70
you were quick..... your reply was quicker than my redaction..
anyway..... Id flowcharted the AC and the little DC I have to work with took rd-bl all the way to pin 5 with the associated voltage drops
RD-BL to Bottom of selenium rectifier 50VAC
Top of selenium rectifier to first 10K resistor -65VDC
After 1st 10K resistor -37VDC
10K POT WIPER (both set to same OHMS) -33VDC
END of both 10K POT wipers -25VDC
Appearances 1,2,22,23 to ground -33VDC
After 1K resistor (all 4 el34's) -33VDC (end of EL34 GRID BIAS CIRCUIT)
anyway..... Id flowcharted the AC and the little DC I have to work with took rd-bl all the way to pin 5 with the associated voltage drops
RD-BL to Bottom of selenium rectifier 50VAC
Top of selenium rectifier to first 10K resistor -65VDC
After 1st 10K resistor -37VDC
10K POT WIPER (both set to same OHMS) -33VDC
END of both 10K POT wipers -25VDC
Appearances 1,2,22,23 to ground -33VDC
After 1K resistor (all 4 el34's) -33VDC (end of EL34 GRID BIAS CIRCUIT)
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°32
Re: Barn find ST70
PeterCapo wrote:jrubin wrote:I removed my dumb question.....
Incidentally pins 4 and 6 on the GZ34 should be read with respect to ground and not each other. reading across 4 and 6 doubles the voltage reading where 4 and 6 to ground shows 360vac
however, if someone could beasure 4-6 with the rectifier removed for resistance (with the power off) id like to know if they match my reading of 61ohms
Jordan
What is your AC mains reading with 360VAC at each of pins 4 and 6 with the rectifier in place? If you have the older transformer, it should be 117VAC. Not sure, but later Stereo 70s might possibly have had a 120VAC primary. So, if your mains read 117VAC or 120VAC with 360VAC at each of pins 4 and 6, your HV secondary is just fine.
To check the DCR of the choke, remove the rectifier tube and just measure across the choke itself.
Hope you are observing safe work practices.
No rectifier and 4EL34's in place and 1 7199
With 360V at pins 4 to ground and pins 6 to ground primary on power transformer shows 115V exactly
I dont want to go higher than 115V input till the rectifier is in place in the hopes that it might load down the circuit when installed. I cant install the rectifier till my capacitors arrive
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°33
Re: Barn find ST70
Look what arrived today
4x 6GH8A
1X OY3
4x 6GH8A
1X OY3
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°34
Re: Barn find ST70
Should I be concerned that the RF choke is .75 henries, or is that a good value?
Guest- Guest
- Post n°35
Re: Barn find ST70
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Last edited by PeterCapo on Thu Dec 03, 2020 10:08 pm; edited 1 time in total
DynakitParts- Posts : 215
Join date : 2008-11-30
- Post n°36
C-354 Choke
A new original Dynaco C-354 choke measures around 1.75 Henries & 62 ohms. Measured out of circuit...
Kevin
Kevin
Last edited by DynakitParts on Tue Oct 06, 2015 9:27 pm; edited 1 time in total (Reason for editing : Bad speller...or is it spellar?)
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°37
Re: Barn find ST70
when i replace the quad cap ill measure the choke and report back
Guest- Guest
- Post n°38
Re: Barn find ST70
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Last edited by PeterCapo on Thu Dec 03, 2020 10:09 pm; edited 1 time in total
Bob Latino- Admin
- Posts : 3260
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Location : Massachusetts
- Post n°39
Re: Barn find ST70
You can measure the resistance of a choke in circuit and it will be much less than an ohm off its actual value. The choke is between the first and second sections of the quad cap. Both of those sections will have a very high resistance (if the quad cap is in good condition) that will have virtually no affect on the resistance measure across the choke. If a choke has a resistance of 62 ohms out of circuit, it will probably measure 61.9 ohms or something like that in circuit ..
Bob
Bob
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°40
Re: Barn find ST70
The quad cap is all but destroyed with 1 reading at 10uf two open and one shorted. I dont recall which points had what, but have a new one on the way
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°41
Re: Barn find ST70
The quad cap has arrived and was installed.... RF Choke measured about 1.55 H out of circuit
Posting movie part 3 tomorrow. 360V on the rectifier showed 117VAC IN.
Further DC circuit testing will happen tomorrow. The EL34's were able to BIAS properly.
117VAC showed exactly 6.4VAC on the heaters. I would doubt the 1 missing 7199 would make any considerable difference on circuit loading
Still waiting on the 7199 adapters and the two new filter caps on the bias circuit
Forgot to take a DC reading of the rectifier tube , ill put that in the next movie
Once I have the replacement tubes for the driver board installed ill start doing some noise testing with shorting plugs and a few 8OHM 100W dummy loads on the output
After that its just signal tracing step by step to ensure everything is in working condition
Then it will be completely disassembled and restored
Posting movie part 3 tomorrow. 360V on the rectifier showed 117VAC IN.
Further DC circuit testing will happen tomorrow. The EL34's were able to BIAS properly.
117VAC showed exactly 6.4VAC on the heaters. I would doubt the 1 missing 7199 would make any considerable difference on circuit loading
Still waiting on the 7199 adapters and the two new filter caps on the bias circuit
Forgot to take a DC reading of the rectifier tube , ill put that in the next movie
Once I have the replacement tubes for the driver board installed ill start doing some noise testing with shorting plugs and a few 8OHM 100W dummy loads on the output
After that its just signal tracing step by step to ensure everything is in working condition
Then it will be completely disassembled and restored
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°42
Re: Barn find ST70
Video 3 posted of the epic saga.....
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°43
Re: Barn find ST70
Somebody tell me what im missing here.
Two 7199 heaters draw .9 (.45x2) amps at 6.4VAC on the heater circuit
Four EL34's draw 6.4 (1.6x4) amps at 6.4VAC on the heater circuit
GZ34 draws 1.9 amps at 5V on the heater circuit
Thats a total heater draw of 9.2 AMPS
Two 7199 heaters draw .9 (.45x2) amps at 6.4VAC on the heater circuit
Four EL34's draw 6.4 (1.6x4) amps at 6.4VAC on the heater circuit
GZ34 draws 1.9 amps at 5V on the heater circuit
Thats a total heater draw of 9.2 AMPS
Guest- Guest
- Post n°44
Re: Barn find ST70
jrubin wrote:Somebody tell me what im missing here.
Two 7199 heaters draw .9 (.45x2) amps at 6.4VAC on the heater circuit
Four EL34's draw 6.4 (1.6x4) amps at 6.4VAC on the heater circuit
GZ34 draws 1.9 amps at 5V on the heater circuit
Thats a total heater draw of 9.2 AMPS
yes, but it is divided over two separate 6.3V filament windings and a separate 5V rectifier filament winding. So you can 'half' the filament current for the EL34 and 7199's, giving you say 3.65A or so for two EL34's and one 7199. I think your transformers filament winding are rated at 4A each, so plenty to spare. So the filament wiring will need to be split up like that, 2x EL34 and 1x 7166 per 6.3V winding.
The 5V filament for the rectifier I think is rated at 2 or 2.5A on the original transformer, not sure about that.
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°45
Re: Barn find ST70
I was looking at this from the wrong angle.... Thats why I shouldnt post before bed
The correct idea was that if tube X's heater uses 1 AMP at 12V. The current burden on the Power transformer on the input given 120V would be 1AMP/10 or 100ma
Using that
7199 heater 6.3VAC .45A @ 117V by [18.57] = 0.024A or 24mA X 2 tubes = 48mA
EL34 heater 6.3VAC 1.6A @117V by [18.57] = 0.086A or 86mA X 4 tubes = 344mA
GZ34 heater 5V 1.9A @ 117V by [23.4] = 0.081A or 81mA X2 tube = 81mA
Total heater current draw at 117VAC = 473mA
The correct idea was that if tube X's heater uses 1 AMP at 12V. The current burden on the Power transformer on the input given 120V would be 1AMP/10 or 100ma
Using that
7199 heater 6.3VAC .45A @ 117V by [18.57] = 0.024A or 24mA X 2 tubes = 48mA
EL34 heater 6.3VAC 1.6A @117V by [18.57] = 0.086A or 86mA X 4 tubes = 344mA
GZ34 heater 5V 1.9A @ 117V by [23.4] = 0.081A or 81mA X2 tube = 81mA
Total heater current draw at 117VAC = 473mA
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°46
Re: Barn find ST70
I should have the new CAPS 100uF tonite. What id like to do is take some measurents of the bias circuit before and after replacement and document the change.
In the old circuit before the changes im going to bring down the bias output reading to about 1.4V at the test point on both sides. just to be sure that any changes make dont underbias the EL34's. Ill also look at the quality of the DC from the selenium rectifier at the capacitors on the oscope.
with this well get a better understanding of what happens if the CAPS in the bias circuit are doubled in value..... I dont predict any significant change (perhaps a slight,slight negative voltage drop) (LESS NEGATIVE VOLTAGE)
Also,
The GZ34 Max current at 350V is 250MA as per the diagram
If the current for each EL34 biased at 1.56V readout is 50ma then the total draw is 200mA
The 7199 tube pulls 9 1.1 and and 12.5 / .35 and 3.5 for all plate and grid currents totaling 26.45mA X2 52.9
This totals 252.9mA
Am I wrong or does the current for this circuit exceed the capability of the GZ34 to provide power ?
In the old circuit before the changes im going to bring down the bias output reading to about 1.4V at the test point on both sides. just to be sure that any changes make dont underbias the EL34's. Ill also look at the quality of the DC from the selenium rectifier at the capacitors on the oscope.
with this well get a better understanding of what happens if the CAPS in the bias circuit are doubled in value..... I dont predict any significant change (perhaps a slight,slight negative voltage drop) (LESS NEGATIVE VOLTAGE)
Also,
The GZ34 Max current at 350V is 250MA as per the diagram
If the current for each EL34 biased at 1.56V readout is 50ma then the total draw is 200mA
The 7199 tube pulls 9 1.1 and and 12.5 / .35 and 3.5 for all plate and grid currents totaling 26.45mA X2 52.9
This totals 252.9mA
Am I wrong or does the current for this circuit exceed the capability of the GZ34 to provide power ?
corndog71- Posts : 840
Join date : 2013-03-19
Location : It can get windy here
- Post n°47
Re: Barn find ST70
zx- Posts : 205
Join date : 2011-08-05
- Post n°48
Re: Barn find ST70
Thanks for your time ..... an posting the info...youtube....vary nice
thanks for the site Bob....................
thanks for the site Bob....................
jrubin- Posts : 85
Join date : 2015-08-28
- Post n°49
Re: Barn find ST70
I dont believe the question was fully understood. As shown below, my math shows that the current draw on the rectifier is close to or exceeds its limit for 350V
The GZ34 Max current at 350V is 250MA as per the diagram
If the current for each EL34 biased at 1.56V readout is 50ma then the total draw is 200mA
The 7199 tube pulls 9 1.1 and and 12.5 / .35 and 3.5 for all plate and grid currents totaling 26.45mA X2 52.9
This totals 252.9mA
Am I wrong or does the current for this circuit exceed the capability of the GZ34 to provide power ?
Bob Latino- Admin
- Posts : 3260
Join date : 2008-11-26
Location : Massachusetts
- Post n°50
Re: Barn find ST70
You are probably correct about the current draw being at the limit of a 5AR4 rectifier BUT > Dynaco used the excellent Mullard 5AR4's which would probably take the 250 milliamp or so current draw in stride. On the VTA ST-70 amps we recommend a 40 milliamp bias setting for each output tube. The 40 milliamp setting is probably easier on today's 5AR4's which are not as "sturdy" as Mullard's 5AR4.
Bob
Bob
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