This project was to see what performance could be achieved with some low cost Russia sweep tubes. I found the 6P13S a tube designed for TV horizontal sweep sections available a low cost. They have a high S of 9.5mA/V and so are easy to drive.
The conservative 14 watt dissipation rating is based on TV service at full power at all times. Looking at the construction of the internal parts I believe a dissipation rating of 20 Watts is more in line with a audio tube service.
I set the plate voltage a 447V and a fixed bias of 13.5 Watts. Plate to Plate loading is 6.6k
Sweep tubes have a few characteristic that can make them challenging to use and achieve good audio performance. The linearity depending on plate loading, voltages and bias levels can be less than ideal for audio service.
The low screen voltages these tube are designed for also make UL operation a bit more complex to get working acceptably.
Finally they tend to be more sensitive to small changes in the screen voltage having a strong effect on the bias current.
They can however deliver high peak plate current levels and so produce surprising power levels. A pair in this design delivers over 33 watt RMS in a 8 ohm load. That is approaching what a EL34 can deliver and yet a NOS 6P13S can be had for a few dollars.
In this design I run the 6P13S in straight pentode mode with fixed bias, Va = 447V, Vg2 = 250V.
Both the low level section B+ and the screen voltages are electronically regulated power supplies for low noise and bias stability.
A pair of 6C10P TV damper tubes was used for the high voltage rectifiers. Two of these tubes deliver 5AR4 performance and in my experience are far more rugged than current production rectifier tubes. When I lost count of how many "new" 5AR4 I have had die I started to look for alternatives. The TV dampers are low cost and full the bill nicely for me.
The driver stage uses a 6F12P. This contains a pentode with very high gain, a whopping S=19ma/V and a triode again with S=19ma/V. The pentode can deliver more gain than almost any small signal pentode I have used. The pentode anode is directly coupled to the triode section's grid. This reduces low frequent phase shift to improve stability. The triode uses a split load to provide the phase inversion signals needed to drive the push pull output tubes. The triode's very high S allows it to do this with little added distortion.
The results were better than I expected with a output power of over 33 watts @0.085% THD. At one watt the THD was 0.024% again giving a pair of EL34 a run for their money.
A small slow speed fan was added as the internal temperature rise was higher than I was comfortable with. The fan noise dissipates at about a 3 feet distance.
The two 6C10P in the upper left corner. A NTC is used to lower turn on serge.
The high voltage rectifiers have snubber networks to reduce noise.
At the bottom is the the module that provides the three regulated power supplies for 380V for the low level section, 250V for screens and -46V for tube bias.
The 380V series regulator and the 250V shunt regulator makes a bit of heat so a heat sink in needed.
It is a bit busy. I would use a larger chassis on a do over
I personally like to look of tubes with top caps. Can you tell? LOL
There are binding posts for 8 and 4 ohms speaker loads
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Notice these pair of 6P13S were not even new and still provided stellar performance.
I wonder how many years they spent in a TV before I got them for $1 each.
1 watt @ 1Khz, 0.024%
Notice how smooth the residual THD is as show on the scope's bottom trace
-3dB 20Khz, THD, 1.03%
Most tube amplifiers struggle at 20Khz due mainly to the output transformer.
Fortunately music program material in is at a pretty low power level at high frequencies diving amplifiers a break.
33.6 watts @ 1Khz, THD 0.085%. I am pretty happy with this outcome.
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