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Amp Build Questions

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Fawkes

New Member
Hi everyone, I am hoping that someone here much smarter than I could help me out on a few questions. I am planning on building a clone of a Fisher SA-16 console amp. I have build a few amps from scratch before with the parts needed already known but I am having some doubts on this one. My first and most pressing question is the power transformer. What voltage for the main should I look for when looking to source out a new power transformer. I am thinking 400 volts with 6.3 for the heaters. I am thinking 400 volts should cover since the highest voltage reference after the power supply is 393 volts but I am not sure if I need to adjust the resistors values or if I am just overthinking this. I am planning on switching out the rectifier tubes for a full wave rectifier setup with diodes to simplify things and make the project easier on the wallet.

My second and third questions are more for curiosity sake. The schematic mentions PTO plugs in a few locations. What are these and why are they there? I have never seen them before on a tube amp schematic. Also, I was wondering what the purpose of the area of the schematic that I circled in red. It falls between the two channels of the amp and seems to interconnect them.
Thanks in advance for any help.
 

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Are you wanting the same B+ voltages shown on the schem.? With SS diodes, the rectified voltage will be higher than with tubes, so you'll need to take this into consideration. IMHO 400v is going to be way too high for the PT secondary. Try to model the PS in PSUD2.

jeff
 
I am trying to keep it close to the same B+ voltage. I didn't think about PSUD2. I will try that. Thanks
 
The area you circled in red appears to be the cathode bias which is bypassed. I'm not sure about the "pto plug" but perhaps it's a test point to measure bias?
 
Maybe look at the Dynaco ST-35 as it is a similar circuit except has UL (ultra linear) and voltages are known plus all parts are available. tubes4hifi etc has the schematic on the their forum and there is a diytube schematic (as they sell a PCB) on their forum that uses a breakdown of the 7247 to 12AU7 and 12AX7 for more tube rolling. Add the EFB mod found on the Tronola Electronics site for longer tube life and improved sonics with 17-18W/CH & lot's of great info there also.

PS trans. http://www.triodeelectronics.com/pa7dynst35sc.html

The integrated Eico HF-81/12 are more the same as that Fisher being Pentode and have lower power 11-12W/CH due to the lower voltages.
PS trans is 280-0-280V.
 
I'm going from memory, but I believe the EZ81/6CA4 has a fairly small voltage drop at the current this amp would need. I think that voltage drop would be about 15 to 20 volts. So if you need 393V DC at the center tap of the output transformers, and assuming the entire amp will pull let's say 130 to 140 mA under quiescent conditions, then you'd need a power transformer with a 320-0-320 secondary at let's say 200 mA.
 
The Dynaco SCA/ST-35 used a 330-0-330 for also a cathode bias amp and did stress the tubes unless modded so maybe closer to 300-0-300 if one is sticking to cathode bias if one is looking for longer tube life. The Eico Amps I mentioned used 280-0-280 and had a great reputation for tube life.

I mentioned the Dynaco ST-35 as it will give you the most power and great sonics plus tube life with the EFB mod. A lot of amps were hard on tubes back in the day to push the power and sonics could be better on the edge with some.

For a non UL PP EL84 diy Fisher clone, Dave Gillespie built one a few years ago on this forum and it used Heathkit AA-100 Output Transformers or similar (Z-565 Dynaco could also be used from the ST-35) May be in the Fisher Forum and had EFB for tube life etc.
 
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ok I am going to ask what is probably a stupid question but I am still new to the science of electronics. If I want my highest output to be 393 volts then why would I start with a secondary lower than that? Wouldn't I want to start higher and then lower the voltage with resistors to match what I am trying to get. How would the voltage increase along the way of the circuit? Sorry, been having trouble wrapping my head around this.
 
And another one to look at is Dave Gillespie's Fisher SA-100 clone.

Fawkes said:
ok I am going to ask what is probably a stupid question but I am still new to the science of electronics. If I want my highest output to be 393 volts then why would I start with a secondary lower than that? Wouldn't I want to start higher and then lower the voltage with resistors to match what I am trying to get. How would the voltage increase along the way of the circuit? Sorry, been having trouble wrapping my head around this.
Click to expand...

It's because AC voltage is specified as RMS volts, but filtering to DC using a large enough capacitor as the first filtering element after the rectifier, the capacitor filters off of the peak voltage. After typical transformer loss, the conversion factor is about 1.3 at rated output current. That's how I got 320-0-320.

The formula I used is:

Filtered DC voltage = PT secondary RMS voltage * 1.3 - rectifier voltage drop.

393 = PT_Secondary_RMS_volage*1.3 - 20.

PT_Secondary_RMS_voltage = (393 + 20)/1.3

PT_secondary_RMS_voltage = 317 (and I rounded up to 320).
 
Now I see where I was struggling. I had not heard of RMS voltage and could not figure out why the voltage would change so dramatically. Thank you.
 
I ran a simulation on PSUD2 using the 320 starting voltage and using the resistor values provided in the schematic. According to the model, it looks like the output voltage is right where I want it to be by using the 320-0-320 transformer. Thanks everyone.
 
Maybe do consider the EFB modded SA-100 amp ( or try on SA-16) mentioned if you want a Pentode Output amp do to the tube life issue and sonic improvement. On many output tubes you can run the screens much lower regulated even though the plates are high with great tube life as Dave has done.
 
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