HH SCOTT 340B 1963.
Hello. I'm fixing this amp.
the owner claims he bought it working.
as he was fooling around with it he dropped it, broke the 7591 and a cap.
the shock also took out the bridge and the 5 watt 20 ohm on the high voltage side.
now replaced and working as I think he got it.
horrible hum increases as volume is turned up.
also finding lots of AC riding on the DC of the 12ax7.
I really don't want to get to deep into this.
I need some voltage specs on the transformer.
looks like the hum is being introduced to the preamp. possibly all coming from the phono section. looking for some hints.
Thanks
DAVE IACOV
t
Hello. I'm fixing this amp.
the owner claims he bought it working.
as he was fooling around with it he dropped it, broke the 7591 and a cap.
the shock also took out the bridge and the 5 watt 20 ohm on the high voltage side.
now replaced and working as I think he got it.
horrible hum increases as volume is turned up.
also finding lots of AC riding on the DC of the 12ax7.
I really don't want to get to deep into this.
I need some voltage specs on the transformer.
looks like the hum is being introduced to the preamp. possibly all coming from the phono section. looking for some hints.
Thanks
DAVE IACOV
t
Investigate further. If the hum is from the phono section, selecting another input should stop it. If not, it isn't from the phono. I would be looking for bad connections around the PSU capacitors.
Schematics at groups.io HHScott group.
Schematics at groups.io HHScott group.
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Thanks
I have been. It's an interesting unit. built in 1963. the wiring is very nice its a piece of art. but a killer to work on. just testing voltages is stressful. right now I'm seeing way to much AC on the DC. About 20 volts at the 4 12ax7. they use resistors not inductors. i might have used a bad bridge ill find out later. still trying to trace and follow the schematic. very tight. I'm pretty sure this is a cap problem I'll keep looking. I'll end up posting all the videos on YouTube. I am going to separate the power amp from everything else and give it a know clean one tomorrow. just to know what I'm up against. I'm pretty sure the power amp section is fine.
Thanks, Dave
I have been. It's an interesting unit. built in 1963. the wiring is very nice its a piece of art. but a killer to work on. just testing voltages is stressful. right now I'm seeing way to much AC on the DC. About 20 volts at the 4 12ax7. they use resistors not inductors. i might have used a bad bridge ill find out later. still trying to trace and follow the schematic. very tight. I'm pretty sure this is a cap problem I'll keep looking. I'll end up posting all the videos on YouTube. I am going to separate the power amp from everything else and give it a know clean one tomorrow. just to know what I'm up against. I'm pretty sure the power amp section is fine.
Thanks, Dave
Attachments
Hi, finding the schematic wasn't hard.
Voltage test points and specs on the transformer is. the schematic shows some voltages but most of it I need to calculate, not a big deal but I'd like to know for sure.
Voltage test points and specs on the transformer is. the schematic shows some voltages but most of it I need to calculate, not a big deal but I'd like to know for sure.
Attachments
Thanks .
I have these schematic. the problem I'm having is finding the specs on the transformers.
Any help would be greatly appreciated
Thanks
Dave
I have these schematic. the problem I'm having is finding the specs on the transformers.
Any help would be greatly appreciated
Thanks
Dave
HH SCOTT 340B
in my previous post I explained how I have been repairing this amp.
An issue has been hum .
my findings are that 20 volts riding on the DC for the 12ax7s.
1, the bridge was bad. replacing the bridge was one issue.
the unit has a 10 ohm then 3, 30 ohm resistors in series before the 4 elements of the 12ax7s.
Also the can capacitor is 75/75 between each resistor. No capacitance lead to the 20 volts ac.
Changing the can was not an option. I added 3000mf and a choke at the bridge
before the first 10 ohm.
this cleaned up the DC, 0 volts ac. and 12.7 volts at each filament.
the hum is all but completely eliminated.
the schematic is located online. Google Hh SCOTT 340B schematic.
download the pdf.
It's a great project
in my previous post I explained how I have been repairing this amp.
An issue has been hum .
my findings are that 20 volts riding on the DC for the 12ax7s.
1, the bridge was bad. replacing the bridge was one issue.
the unit has a 10 ohm then 3, 30 ohm resistors in series before the 4 elements of the 12ax7s.
Also the can capacitor is 75/75 between each resistor. No capacitance lead to the 20 volts ac.
Changing the can was not an option. I added 3000mf and a choke at the bridge
before the first 10 ohm.
this cleaned up the DC, 0 volts ac. and 12.7 volts at each filament.
the hum is all but completely eliminated.
the schematic is located online. Google Hh SCOTT 340B schematic.
download the pdf.
It's a great project
That's also the negative bias source for the output tubes. Did you check and make sure it's correct?
I don't see why you think you need transformer specs beyond the schematic to track down a hum problem. In any case you're not going to find them.
why you ask. maybe it's because it's old.
But mainly I was only getting 48.5 volts. doing the math that left me with 8.5 volts dc on each of the 4, 12ax7s.
now the transformer wasn't the problem but having specs on the items I work with. well it's just a good idea.
But mainly I was only getting 48.5 volts. doing the math that left me with 8.5 volts dc on each of the 4, 12ax7s.
now the transformer wasn't the problem but having specs on the items I work with. well it's just a good idea.
Whenever you have no specs on the power transformer you can use a variac to dial in the main voltage with all tubes inserted and measure the AC voltage on filament winding until it hits 6.3VAC. Once stabilized then measure the high voltage windings and bias windings and other extra windings. Write down all the voltage points, AC and DC. Normally you get in the range between 105-115VAC on the primary because most vintage gears were expected to be powered below 120VAC. Of course this should be done AFTER you slowly reformed all the filter caps with the variac.
All true under normal conditions. the 4 12ax7 are in series and tap 9 not used. the bridge was low. the bridge was bad.
replaced the bridge. ac voltage about 58 not sure its all wired in now. dc voltage was low about 50 volts . not knowing if the 58 was correct is why I needed specs.
so voltage on the 12ax7 was low and high ripple.
all of the can caps were bad. open.
should be 75/75 each.
made a new filter for the DC.
DC now at proper voltage and the voltage on the 12ax7 is about 12.23 each. all previous hum from the preamp is gone.
A very slight hum on the output still exists but it's very slight and doesn't increase with volumes.
I eliminated the tape out connections and are using them as a power amp in source. I'll make another video after it's finished. remember not all tubes have been replaced yet. biases have not been adjusted. I'm waiting for new caps to come in and 2 new 6u8. and 2 more 7591. about 60$ in parts not counting the tubes. the owner bought them. I have a countless number of hours into this ,mostly reading, it has been a lot of fun. this is the 4th tube amp I've worked on. I use a hantek 2d72 and a few other dmm for everything I'm doing.
here is the link to the first video
Thanks for the input it is greatly appreciated
Dave
replaced the bridge. ac voltage about 58 not sure its all wired in now. dc voltage was low about 50 volts . not knowing if the 58 was correct is why I needed specs.
so voltage on the 12ax7 was low and high ripple.
all of the can caps were bad. open.
should be 75/75 each.
made a new filter for the DC.
DC now at proper voltage and the voltage on the 12ax7 is about 12.23 each. all previous hum from the preamp is gone.
A very slight hum on the output still exists but it's very slight and doesn't increase with volumes.
I eliminated the tape out connections and are using them as a power amp in source. I'll make another video after it's finished. remember not all tubes have been replaced yet. biases have not been adjusted. I'm waiting for new caps to come in and 2 new 6u8. and 2 more 7591. about 60$ in parts not counting the tubes. the owner bought them. I have a countless number of hours into this ,mostly reading, it has been a lot of fun. this is the 4th tube amp I've worked on. I use a hantek 2d72 and a few other dmm for everything I'm doing.
here is the link to the first video
Thanks for the input it is greatly appreciated
Dave
I would use a 10 ohm resistor for each 7591 cathode to ground for easy bias and balance check on the test points, instead of a single 3.3 ohm resistor (R47 & R147) for both tubes in each channel. The four resistors can also act as fuses in case of catastrophe to protect the output transformers.
Thanks
So take the 3.3s out? and put the 10s in? what should I see on the test points. ? I haven't looked this up so I'm not familiar yet.
thank you for the pointer.
Dave
So take the 3.3s out? and put the 10s in? what should I see on the test points. ? I haven't looked this up so I'm not familiar yet.
thank you for the pointer.
Dave
Each resistor indicates how much current the power tube plate (plus screen) is drawing. 7591 total plate dissipation is 19 watts (total screen dissipation is 3.3 watts but let's just use plate only just to be conservative) since plate is 450V so 19/450 = 42mA. So you do not want to bias each tube exceeding 42ma or 10Ω x .042 = .42V on cathode. 10 ohm is just easier math without calculator. In stock circuit with 3.3 ohm for BOTH tubes, 3.3 x .084 so you don't want to exceed 0.2772V at the test point. Having separate resistors for each tube is just easier to balance bias.
Thanks. changing to 10 ohms is easier on the math no problem.
I get the math and understand what you are saying.
although I'm able to understand and interpret the schematic make the repairs with confidence. I don't understand the biasing completely yet. still reading.
Thanks for taking the time and explaining this.
what would be the issue with just staying with the 10 ohms vs the 3.3?
Dave
I get the math and understand what you are saying.
although I'm able to understand and interpret the schematic make the repairs with confidence. I don't understand the biasing completely yet. still reading.
Thanks for taking the time and explaining this.
what would be the issue with just staying with the 10 ohms vs the 3.3?
Dave
3.3 ohms for two tubes' cathodes tied together would be 6.6 ohms for each tube. You can of course use 6.6 but why when 10R is readily available and easier math. Unless the test point voltage is labeled on the chassis at a specific value based on 3.3 calculation but you can always relabel the value with 10 ohms. Dynaco ST70 chassis is label with a specific voltage for bias, for example. Also, make sure the resistors are precise and highly matched so you won't get misreading of the bias.
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3.3 ohms for the 2 tubes tied together r47 replace with the 10 is fine, im missing something when you mentioned the 6.6 ohms. the 10 makes more sence to me any way you want to look at it.
I'm looking forward to my next repair.
the 4 that I've done so far were all, well they just happened.
it's been 50 years since I've worked on anything point to point.
and even with your advice I a tad lost but not so lost that I feel well confused.
only at the point as to where I simply need a little more understanding if that makes any sence.
Thanks for all your advice im sucking it up like a sponge.
I'll keep working on it
Dave
this is the section you are talking about , correct. 1, 3.3 ohm,
I'm looking forward to my next repair.
the 4 that I've done so far were all, well they just happened.
it's been 50 years since I've worked on anything point to point.
and even with your advice I a tad lost but not so lost that I feel well confused.
only at the point as to where I simply need a little more understanding if that makes any sence.
Thanks for all your advice im sucking it up like a sponge.
I'll keep working on it
Dave
this is the section you are talking about , correct. 1, 3.3 ohm,
