In this case and if the filter capacitors aren't knitted to the edge I'd omit the belts (5AR4) and leave the braces (Si diodes).
Best regards!
The whole amp is pushed close to the edge. Monoblock them. Use the heater power for one channel to run a pair of 6AU4 in place of the missing channel's EL34. Second chassis, and second PA060 power Iron and you're good. The 6AU4 will drop the voltage a wee bit, but that is feature, not bug... 🙂 Better yet, use the second OPT spot for building a choke-input filter.
Douglas
Douglas
Also
@Bandersnatch
@jxdking
With Both channels driven to full , the circuit pulls 500ma current, vintage GZ34 is totally streched in this scenario and a modern one won't make it . Please read this.
https://audiokarma.org/forums/index.php?threads/dynaco-st-70-base-line-testing.578485/post-7609452
The capabilities of a valve amp circuit and opt's are Fully realised only if the power supply is capable of delivering the demanded voltages and current instantly and cleanly. The ST 70 is a budget amp and there are compromises in the power supply. We are fully aware of them and they have been addressed in the following manner.
First it should be noted that this amp is located in India and the electricity supply standard here is 240V/50Hz . The original power transformer is designed for 117V/60Hz . I could have used a step down but that won't solve the issue of running a transformer designed only for 60Hz at 50Hz. The already marginal capacity of the transformer shall further suffer at lower supply frequency. So a completely new transformer was built with primary rated for 250 volts ( supply voltages go above 240 here) and 50% higher current capacity at 50 Hz compared that of the original at 60Hz. The choke was also replaced with a new one of similar higher capacity. All new caps were fitted inside the chassis, the can cap is there now only for appearances sake. After this , the final stumbling block was the rectifier. It is a bottleneck in the the original design as shown inthe link above and with the upgrade of all other ps components it would have been more like that. Hence the yellow sheet mod was resorted to. The idea here was to get the best out of the original design, overcoming cost cutting measures done for commercial reasons without straying too far away from the design concept. I think we have achieved that goal. The amp plays beautifully clean even at loud volumes with superb dynamics.
For monoblocks , I don't need to hack this. I have a pair of Klangfilm cinema amps in the projects que. They won't get the yellow sheet mod when we work on them as 2xEL34 and the associated small signal valves is an easy peasy load , even for a current production GZ34.
Thanks and Regards,
Anwesh
Tube amplifiers, intended for playing music, need not play sine waves continuously at full rated power.
I have seen hundreds of Stereo 70s with the original Mullard GZ-34 rectifier still in fine condition after many years.
I don't have a good quad of EL34 , until that arrives , we have installed Sovtek 5881. There are one or two changes made for that.
Output has been adjusted to 25wpc. The amp is running very stable and sounds fantastic. The power transformer doesn't go beyond warm ( can keep hand on it) even after several hours of playing .
Regards,
500mA may be a peak transient that is supplied by the capacitor when needed, but in any case the 5AR4 has a peak current rating of 750mA per plate. If the amp is run at full power, it's rated continuous full power, and that was 5oomA, the choke would be destroyed as it is rated for 200ma. All of the total hullaballoo about the amp overpowering the 5AR4 is BS, (poor quality new issues from China or Russia not withstanding). My JJ has been working with no issue and I use them in my other 2 amps accordingly. Any changes away from the OEM circuit can expect to suffer the fate of DIY engineer/modder disease.
I must admit I do not understand the note concerning the destruction of a 200ma rated choke by applying 500 ma through it. What is the failure mode? I will point out that inductance is controlled by voltage vs permeability and heating is controlled by current flow. Modern materials are drastically under rated by safety agencies and a choke that is an instant "ouch" from a single finger test touch is only operating at it's max temperature rise of 90 C in a test ambient temperature of 40 C. The wire involved, class B, is NEMA rated for 155 C or class F operating temperature. UL, CSA, CE all derate this limit to class B ,130 C total temperature. These ratings are intended for 100% duty cycle forever conditions. I will admit that the choke would be desperately hot and destructive to it's local signal and power components but I fail to understand it's destruction, assuming of course that it was designed and constructed by a mfg who actually knew what temperature rise in an ambient meant and what circular mils of wire surface vs current draw might mean to open flames arising.