My latest 'find' in the free listings locally....
🙂
Admiral HiFi Stereo
Chassis # 8YICX
SE EL86
Good-looking power transformer.
Output transformers are tiny so a rebuild into something basic will be in order, I guess.
Interesting that the 2nd (stereo) OT is screwed on to the back of the chassis - I've seen this before and assume that it was a chassis for a mono hifi that got adapted for stereo when that became the 'latest thing'.
I'm not going to keep the radio or think about 'restoring' it to its former glory - I got it before its next stop - the landfill.
Circuit ideas?
🙂
Admiral HiFi Stereo
Chassis # 8YICX
SE EL86
Good-looking power transformer.
Output transformers are tiny so a rebuild into something basic will be in order, I guess.
Interesting that the 2nd (stereo) OT is screwed on to the back of the chassis - I've seen this before and assume that it was a chassis for a mono hifi that got adapted for stereo when that became the 'latest thing'.
I'm not going to keep the radio or think about 'restoring' it to its former glory - I got it before its next stop - the landfill.
Circuit ideas?
Attachments
The 6CW5/EL86 looks like a decent enough "12" W. type. The "recipe" is more a less standardized. 12AX7 section voltage amplifier into full pentode mode "final". Loop NFB of some kind is mandatory. Given the puny O/P "iron", short loop, as opposed to global loop, seems right. Regulate g2 B+, for max. open loop linearity.
I suspect you'll find a selenium bridge rectifier "below decks". That must go. It seems a PSU filter choke is present.
I suspect you'll find a selenium bridge rectifier "below decks". That must go. It seems a PSU filter choke is present.
Thanks, Eli.
Yes, nice to find a choke in there....
I expect I'll find the B+ fairly low as it seems the EL86 is a lower-voltage, higher current tube (compared to the EL84), using a lower impedance OT.
I'm also curious about those OTs - the speaker lineup seems to be 3-4 ohm load - the manual specifies a 3-4 ohm speaker if one side is to be disconnected and replaced with an extension speaker ("for better stereo separation"...those 60s ping pong games and steam engine stereo sound effects LPs needed that.... :0 ).
So the OTs may be the usual EL84 type if used with nominal 8 ohm speakers.
EL86 tubes are pretty cheap, so that's appealing.
Screen regulation - do I need something complicated or just a gas regulator tube or even a zener like the RH84v2 uses?
I don't understand the short loop NFB concept (yet).
I'd be interested in seeing if the Kitic RH idea would translate well to EL86.
I also found a schematic (with tube screen regulator?) from Lagarto.
Yes, nice to find a choke in there....
I expect I'll find the B+ fairly low as it seems the EL86 is a lower-voltage, higher current tube (compared to the EL84), using a lower impedance OT.
I'm also curious about those OTs - the speaker lineup seems to be 3-4 ohm load - the manual specifies a 3-4 ohm speaker if one side is to be disconnected and replaced with an extension speaker ("for better stereo separation"...those 60s ping pong games and steam engine stereo sound effects LPs needed that.... :0 ).
So the OTs may be the usual EL84 type if used with nominal 8 ohm speakers.
EL86 tubes are pretty cheap, so that's appealing.
Screen regulation - do I need something complicated or just a gas regulator tube or even a zener like the RH84v2 uses?
I don't understand the short loop NFB concept (yet).
I'd be interested in seeing if the Kitic RH idea would translate well to EL86.
I also found a schematic (with tube screen regulator?) from Lagarto.
Attachments
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Vintage console speakers, like tube radios, have 3 or 4 ohm impedence. If your speakers impedence is 8 ohms, the output transformer reflected primary impedence will be the usual 5K, and the EL86 will not be the best tube. B+ should be 200V on this amp with the selenium rectifier and the nominal 110V primary, but you will use a silicon rectifier instead, and primary voltage is likely higher. You may get 250V, a good choice for EL84/6BQ5. Check if the power transformer does have hum/vibrations. They did not care back at the time, on this consumer grade audio gear.
By the way, this radio does have nice knobs, mostly undamaged. After a bath in the ultrasonic cleaner they may look nice again. I would also keep the speaker cloth before the trip to the landfill. It looks perfect for guitar amplifiers. The variable capacitor may be used to select the best global feedback capacitor value to put across the resistor.
By the way, this radio does have nice knobs, mostly undamaged. After a bath in the ultrasonic cleaner they may look nice again. I would also keep the speaker cloth before the trip to the landfill. It looks perfect for guitar amplifiers. The variable capacitor may be used to select the best global feedback capacitor value to put across the resistor.
Pcan-
Thanks for those comments.
I'll check out the power supply later today to see about the rectifier situation and the possible B+, and also PT hum. I hadn't thought about using the variable cap in that way. Once I get it disconnected, I'll see if I can measure its capacitance easily. Up to now I've been using a capacitor substitution box or just tacking (with solder) different small caps in place or just using test leads to clip on to different caps.
Thanks for those comments.
I'll check out the power supply later today to see about the rectifier situation and the possible B+, and also PT hum. I hadn't thought about using the variable cap in that way. Once I get it disconnected, I'll see if I can measure its capacitance easily. Up to now I've been using a capacitor substitution box or just tacking (with solder) different small caps in place or just using test leads to clip on to different caps.
Curious that the on-off switch is combined with the Treble control. I wonder what the thinking behind that was..
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A single 0A2 serving both channels is fine and it will plop right in, if the chassis is recycled.
The Lagarto schematic is interesting. The 6CG7 section is serving as a replacement for a "missing" UL tap on the O/P trafo. UL mode, without some additional loop NFB, is shaky in the damping factor dept. With the right speaker (not sealed box) that setup could be just what the Dr. ordered, in combination with the low mass O/P "iron" present. Adding a GNFB loop raises the issue of core saturation.
A quick update- The radio and audio sections are working -needed some new tubes, and the controls are scratchy -probably because of leaky caps or just disuse. So the OTs are good. The B+ was 120 volts, but it wasn't clear to me if that was because of leaky filter cap or if that was the design. Single diode 'top hat' rectifier(half wave).
The HV secondary on the PT is about 160 V AC without a load. No CT on the HV secondary.
Next steps : Measure the OTs and off to PSUD to look at possibilities.
Output transformers are 5K6 into 8 ohms (2k8 into 4 ohms ) Turns ratio about 26.5, Impedance ratio about 700
Choke is 2H 52 ohms
PSUD shows about 200 volts B+ with a SS bridge.
This looks to me like EL86 voltage range vs EL84? But the OTs are better matched to EL84 if 8 ohm speakers are being used.....
Voltage doubler?
Perhaps a voltage doubler & dropping resistor would get me to EL84 territory? (~325V) It's not clear to me how dramatic the (bad) effect would be if I ran the EL86 tubes into a 5K6 secondary on the output transformer.
The HV secondary on the PT is about 160 V AC without a load. No CT on the HV secondary.
Next steps : Measure the OTs and off to PSUD to look at possibilities.
Output transformers are 5K6 into 8 ohms (2k8 into 4 ohms ) Turns ratio about 26.5, Impedance ratio about 700
Choke is 2H 52 ohms
PSUD shows about 200 volts B+ with a SS bridge.
This looks to me like EL86 voltage range vs EL84? But the OTs are better matched to EL84 if 8 ohm speakers are being used.....
Voltage doubler?
Perhaps a voltage doubler & dropping resistor would get me to EL84 territory? (~325V) It's not clear to me how dramatic the (bad) effect would be if I ran the EL86 tubes into a 5K6 secondary on the output transformer.
Attachments
That fits a rip the purchaser off theme, just like the puny O/P "iron". 🙁
The power trafo looks like the best part of the whole shebang. So, definitely a voltage doubler with 2X UF4007s. Use modest valued caps. (say 15 μF.) in the doubler stack and avoid power losses to heat in a substantial dropping resistor. Follow the doubler stack with the choke and a substantial cap., for ripple suppression. Now, a final RC section for the B+ reservoir.
8 Kohm primaries are "typical" for EL84s. FWIW, I like the 6CW5 with that 5.6 Kohm stuff. Don't worry about the max. anode voltage specification, provided you RELENTLESSLY hold plate dissipation just below 12 W., along with 0A2 ("150" V.) regulating g2 B+.
It looks like I have some options, then, if the 5k6 OTs would work with the 6CW5.
With those (very) small OTs, I don't know if I would be better sticking with the RH circuit (and probably EL84s, and the voltage doubler) or trying the RH idea with 6CW5s and lower 200 v voltage from a bridge) or something else completely.
If I have the screen voltage a lot lower than the anode, won't that drop the power output? There's not much margin to play with!
I got the chassis cleaned out pretty much. The tube sockets were really wired 'well' (all the leads well wrapped, so unsoldering seemed to be a recipe for breaking the fragile lugs) and the heaters were using the chassis for one side of the wiring, so I ended up taking out the sockets as well. Only a few dollars for new sockets, and a lot faster. Also, I think I'll be able to put both the OTs on top of the chassis and get the tubes in a more symmetrical layout.
I want to re-use some of the pots, probably, so unsoldering the wiring on those will take a while.
Well, they certainly put the ^&*&^ cabinet together well - like most of the ones from that era that I've had to dismantle. 🙂 Lots of tenon joints with many screws - during assembly they glued it and the screws held everything tight until the glue set. And this one had no particle board - all plywood and solid wood. It almost made me nostalgic for IKEA 'furniture' - no problems knocking that stuff apart!!
No, power production is not reduced. Power O/P is current flowing in the load (P = I2R). The plate current in pentodes is primarily controlled by the g1 and g2 potentials, with a small effect by the plate potential. The 6CW5 datasheet I previously linked shows the plate curves with Vg2 at 170 V. For any given g1 potential, what you get with Vg2 at "150" V. and a "tall" anode potential will be similar to what you get with g2 at 170 V. and a "short" anode potential.
Perhaps a long, but non-global, NFB loop is the way to go. Cap. couple the plate of the "final" to a voltage divider feeding the voltage amplifier's cathode. With this option, there is no linearization of the O/P trafo, but concerns about core saturation are absent too.
I see that often,,, moves the AC power off the volume pot, if a separate on-off switch isn't in the budget... My guess, anyway!!!
On this console, the volume pot is a 'loudness' control with extra lugs and a lot of tone wiring attached. Also, the treble control is furthest from the function switch (with the low level signals from phono, etc), so perhaps keeping the AC far away was part of the thinking. Certainly the power switch on the volume feels more 'natural' if you are accustomed to a 50s table radio. It also avoids having the unit power up with 'too high' volume (if the kids or deaf grandpa had been the last to use the set...).
It is interesting to try to figure out what the designers/engineers/techs were thinking...on this chassis the power transformer is next to the tuning capacitor. The PT is 'tilted' away from the variable cap with a couple of thick washers under one side. Perhaps there was a hum problem?
How difficult would it be to 'optimize' the RH84 circuit for the 6CW5?
I'm thinking of 200v (approximately) B+, 150v screen (gas regulator tube), with the LM317 regulator to adjust the current - most of the Kitic circuit.
12AT7 or 12AX7 for the input/driver tube.
OT reflecting 5k6 from nominal 8 ohm speaker
I have a scope and audio generator, etc...
It sounds like adjusting the feedback resistor and tube currents/bias are the main 'variables'?
I'm thinking of 200v (approximately) B+, 150v screen (gas regulator tube), with the LM317 regulator to adjust the current - most of the Kitic circuit.
12AT7 or 12AX7 for the input/driver tube.
OT reflecting 5k6 from nominal 8 ohm speaker
I have a scope and audio generator, etc...
It sounds like adjusting the feedback resistor and tube currents/bias are the main 'variables'?
You should really try the output transformers, unless you have a backup plan ready if you find that quality is not good enough. I have got many 3-5W SE output transformers of this type and size back in the '80 when people were throwing away that kind of stuff. Most of them are basically a small power transformer in disguise, with a random gap in the laminations. A few are built properly, but unfortunately none of them came from the cheaper "low B+" designs with 2-3K primary. They were always paired with EL84/ECL82/ECL86 tubes and 3 ohm speakers. This means 10K load with 8 ohm speakers, a difficult proposition for most output tubes. Your 5k6 ohm / 8 ohm OT truly is a lucky find, if it works properly.
Nice project.
The Garrard Type A is a serious effort and was TOTL. The outer heavy cast platter is taken out for inspection or MIA?
Regards,
The Garrard Type A is a serious effort and was TOTL. The outer heavy cast platter is taken out for inspection or MIA?
Regards,
Turntables are quite common on the used listings here, so I'm afraid this one is headed for the steel recycling bin.
I recall the Garrards from the 60s then Dual TTs appeared for 'consumers', then all the Japanese TTs. Before that happened, I'd moved away from vinyl. I 'flirted' with vinyl a few years ago, then realized I had left for good in the 90s.
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Quick update - I've breadboarded 'something like' the RH84 with the 6CW5/EL86, ~200v B+ (from the bench supply), screen fed via a 39v zener, 12AT7 driver.
It's making sound/music but needs work to get the gain/power 'adjusted'.
The BK bench supply that I was using for the screen has decided to trip its overload (error), so that's why I switched to the zener. Another one for the 'project list'. Fortunately I have another bench supply (Heathkit) which does work.
The fun continues.... 🙂
It's making sound/music but needs work to get the gain/power 'adjusted'.
The BK bench supply that I was using for the screen has decided to trip its overload (error), so that's why I switched to the zener. Another one for the 'project list'. Fortunately I have another bench supply (Heathkit) which does work.
The fun continues.... 🙂
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I've been trying some different amounts of NFB from the 6CW5 plate to a voltage divider on the cathode of the 12AT7.
Watching the results on the scope (looking at the grid signal on the 6CW5) and measuring the THD after the 12AT7 (at power tube grid also).
With no feedback I get the least THD (using HP339A)...this seems strange to me.
Tomorrow I'll measure the THD at the output into the load resistor.
I'll post a schematic tomorrow.
For now:
12AT7 Cathode bias 1.6v, cathode resistor(s) total 500 ohms, plate voltage 115 v, plate resistor 27k ohms
6CW5/EL86 Plate-Cathode voltage 178v, current 70mA, plate dissipation 12.5 W
Screen connected to B+ via 39v zener
Power out to ~8ohm resistor load 3.3 watts audio
NFB - not much 1.7dB @ 6% THD
NFB- none 5.5% THD
Watching the results on the scope (looking at the grid signal on the 6CW5) and measuring the THD after the 12AT7 (at power tube grid also).
With no feedback I get the least THD (using HP339A)...this seems strange to me.
Tomorrow I'll measure the THD at the output into the load resistor.
I'll post a schematic tomorrow.
For now:
12AT7 Cathode bias 1.6v, cathode resistor(s) total 500 ohms, plate voltage 115 v, plate resistor 27k ohms
6CW5/EL86 Plate-Cathode voltage 178v, current 70mA, plate dissipation 12.5 W
Screen connected to B+ via 39v zener
Power out to ~8ohm resistor load 3.3 watts audio
NFB - not much 1.7dB @ 6% THD
NFB- none 5.5% THD
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