Jolida schematic questions

[JayH3]

I plan to copy the Jolida SJ-502 design.

I was studying this schematic - http://www.drtube.com/schematics/jolida/sj-502a.gif

The resistor between the pot labeled V2 and ground does not have a label. Does anyone know what it could be?
Also, I can't locate R19 and R20. I figure one of them is the above mentioned, but what about the other?

What are recommended upgrades to this amp?

Thanks in advance.

 

[euro21]

Example:

Coupling capacitors: Sonicraft Sonicap .22uF/600v, Audience AuriCap capacitors
* Signal Ground capacitors: Sonicraft Sonicap .47uF/600v
* Power supply filter capacitors: upgraded to higher capacitance/low ESR pieces, exmpl.: Blackgate WKZ
* Power supply bypass capacitors: Sonicap high voltage units
* Signal path resistors: Dale/Vishay 1 watt 1% non-magnetic Metal Film (x24 pieces)
* Bias Circuit resistors: Dale/Vishay 1%, 3 watt resistors used in the bias cicuit to allow for either EL34 or 6550 output tubes.
* Grid stopper resistors: Japanese Riken Ohm Carbon Film units solder directly at the tube pins (x4 pieces)
* Tube emitter cushion resistors: Dale/Vishay 5 watt 1%, non-magnetic, or Mills 5W
* Dale/Vishay 3 watt 1%, non-magnetic resistors utilized in the bias stage for lower noise
* Diodes: Fairchild "Stealth" rectifying diodes are utilized in the rectifying stage of the power supply.
* UF4007 diode utilized in the bias power supply
* Volume control: Noble or Carbon based pot installed
* Source selector: Grayhill unit is installed for it's quality and longevity.
* Internal wiring: Quality 22 gauge copper wiring
* RCA input jacks: Replaced with high quality Gold plated pieces - x8 pieces
* Balance control bypassed
* Power tubes: Svetlana or JJ EL34 or Sovtek 6550WE (Metal Base) or one set of each.
* Driver tubes: Quality 12AT7 driver tubes, or 12BH7 Sylvania
* Input tubes: Quality 12AX7 input tubes, or 6072A Amperex

 

[audiowize]

The Pot V2 is in the other channel, so I wouldn't worry about what's over there. It is odd that there's no R19 or R20. I would replace R6 with a current source, the .22uf caps with V-Cap teflons, the 100k pot with a PEC pot, R13/R16 with AN tants, etc.

 

[SY]

You'd do better fixing the circuit faults than splashing for designer parts which will make close to zero difference in the sonics.

The SRPP is a poor choice for input stage topology; I'd certainly redo that. The diff amp is a very nice 1957 design, but with modern parts (CCS in the tail and matched plate resistors), you can probably drop the distortion noticeably.

V2 is set up as a balance control. The unmarked resistor could be something like 10k- it's not critical, it sets the degree to which you can unbalance things.

 

[audiowize]

Fixing a driver like that which is built out on a PCB would be quite a pain. I'd imagine you could snip one of the pins on the 9 pin socket to heat 1/2 the tube, make R2 a .1uf cap, and put a plate load resistor in between the plate of the top 1/2 and the .1uf cap to load the driver tube. At that point, you could ditch the 12AX7 in favor of something that actually sounds good... or you could buy a different amp.

 

[pchw]

The diff amp is a very nice 1957 design, but with modern parts (CCS in the tail and matched plate resistors), you can probably drop the distortion noticeably.

Hi SY,

Do you mean use matched plate resistors when CCS is in the tail?
Most of the schematics that I saw had different plate resistor values, often off by more than 10%, e.g. Eico seemed to always use 33K and 28K with 18K in the tail.

Thanks,

 

[JayH3]

Thanks for everybodys responses, they are helpful.

To clear up some confusion, I dont actually own this amp. I heard decent things about it and the schematic looked like a fairly simple circuit so I figured I would try building it, and while I was at it use the better components that people ussually upgrade to.

If anyone has any other suggested amp schematics I am open. Looking for 30+WPC, clear tight bass, good imaging, etc...

 

[audiowize]

What's your budget?

 

[SY]

Do you mean use matched plate resistors when CCS is in the tail?
Most of the schematics that I saw had different plate resistor values, often off by more than 10%, e.g. Eico seemed to always use 33K and 28K with 18K in the tail.

Exactly. The unbalance in the Eico circuit is because the 18k resistor only approximates a CCS.

 

[JayH3]

What's your budget?

I would like to keep it between $300-700.

 

[audiowize]

For making things easy, I would consider the DIY tube Poseidon driver boards, and use a 5751 instead of a 12AX7 at the input. There are tons of driver designs out there, this one is just a great time saver and definately in your budget. A Dynaclone ST-70 power transformer, a solid state power supply, and a pair of Magnequest MQ-470's will round out a pretty nice amplifier. Take care to build a fairly nice power supply to get the most out of it. I would emphasize not cheaping out on the power transformer or outputs.

-Paul

 

[Cycline3]

You'd do better fixing the circuit faults than splashing for designer parts which will make close to zero difference in the sonics.

The SRPP is a poor choice for input stage topology; I'd certainly redo that. The diff amp is a very nice 1957 design, but with modern parts (CCS in the tail and matched plate resistors), you can probably drop the distortion noticeably.

V2 is set up as a balance control. The unmarked resistor could be something like 10k- it's not critical, it sets the degree to which you can unbalance things.

SY nailed this one. Especially if your budget is $700. Get some real iron, nice tubes and nice (not necessisarily expensive) capacitors for the signal path. The money is much better spent there than on expensive tid bits. Also, if you didn't use designer parts and built the schematic exactly as shown, it would still likely be the best $700 amp you've heard! One of the DIY benefits. And if you want to invest the research time and ask some questions here about revising the circuit, I'm sure you will find lots of people who will help you.

 

[JayH3]

I would like to do circuit modifications or new designs one day, but since I am new to tube amp design I want to start with a design I know will work and then go from there experimenting.

BTW, what would be a good choice for output iron? Hammond, Edcor? I have heard good things about Magnequest but they seem too expensive for my first project.

 

[audiowize]

It would be foolish not to use a different design. Look around for a Williamson or something similar. I'm not sure why you're not willing to just use boards meant for the Dynaco MK-IV monoblocks.... you get documentation and a VERY proven design. The Chinese have no f-ing idea of what they're doing.