Help identifing what class it is, what wattage expected from circuit?

Its just not real high res. Maybe the limit of the camera, or something is getting shrunk in the upload. Even without blowing it up, it looks kind of fuzzy.

I can't find that brand or model in the Sams index either, so thats no help.
 
Ok, thanks for your help so far, let me try re-taking some pictures, I will try some close --ups better lighting, maybe in two or three sections and repost pictures will help. The schematic is like 17" X 5", in bad shape with stains. May not get to it tonight, please check back soon.
Thxs
 
My old schematic of this unit correctly states the upper number is the "operating voltage", while the low number is the "standby voltage". Other than that, I have no literature, with a schematic that isn't even as good as yours.

My information dates to June of 1980 when I came across one of these sans power transformer. My notes show I called Executone at the time, and through a discussion with one of it's representatives, was told that it was rated at 75 watts.

The OPT is strange however. At the time, I measured a turns ratio from the primary at the 20 volt secondary of 21.73. based on a 75 watts rating, that makes this tap a 5.333Ω tap, with a reflected impedance of 2518Ω. The 70 volt secondary had a turns ratio of 7.01, with a 75 watt rating making that tap a 65.333Ω tap, with a reflected impedance of 3214Ω. I could never tell from my poor copy of the schematic what the 3 volt winding was (3.3Ω? 3.5Ω?), but it has a turns ratio of 182.08. If it is a 3.5 volt secondary, that makes it a .16333Ω tap (appropriate for 50 8Ω speakers in parallel), with a reflected impedance of 3777Ω. But all of these are too low for KT88 tubes under100 watt operating conditions. Just to make things even more confusing, I show that the transformer in my unit had UL taps at 26.6% of the winding as well. But that was all a long time ago.

Remember that even when 6550/KT88 tubes are operated in "100 watt" mode, assuming that all the power supply voltages hold up, you'll still only get about 81 watts of power into a secondary load due to OPT losses. So, it is a certainty that the unit is not a 100 watt unit.

If you could specify what the heck that 3 volt winding is actually rated at from your schematic, I could at least put that mystery of this unit to bed -- after what, 37 years?? :)

Dave
 
My old schematic of this unit correctly states the upper number is the "operating voltage", while the low number is the "standby voltage". Other than that, I have no literature, with a schematic that isn't even as good as yours.

My information dates to June of 1980 when I came across one of these sans power transformer. My notes show I called Executone at the time, and through a discussion with one of it's representatives, was told that it was rated at 75 watts.

The OPT is strange however. At the time, I measured a turns ratio from the primary at the 20 volt secondary of 21.73. based on a 75 watts rating, that makes this tap a 5.333Ω tap, with a reflected impedance of 2518Ω. The 70 volt secondary had a turns ratio of 7.01, with a 75 watt rating making that tap a 65.333Ω tap, with a reflected impedance of 3214Ω. I could never tell from my poor copy of the schematic what the 3 volt winding was (3.3Ω? 3.5Ω?), but it has a turns ratio of 182.08. If it is a 3.5 volt secondary, that makes it a .16333Ω tap (appropriate for 50 8Ω speakers in parallel), with a reflected impedance of 3777Ω. But all of these are too low for KT88 tubes under100 watt operating conditions. Just to make things even more confusing, I show that the transformer in my unit had UL taps at 26.6% of the winding as well. But that was all a long time ago.

Remember that even when 6550/KT88 tubes are operated in "100 watt" mode, assuming that all the power supply voltages hold up, you'll still only get about 81 watts of power into a secondary load due to OPT losses. So, it is a certainty that the unit is not a 100 watt unit.

If you could specify what the heck that 3 volt winding is actually rated at from your schematic, I could at least put that mystery of this unit to bed -- after what, 37 years?? :)

Dave

Dave, may bad on the top/bottom voltage reading thanks, I corrected that.
Ok, I just check schematic and it shows 3.15v on one side of the center tap, and 2.9v on the other side. Wow 37 year mystery solved.
Rick
 
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Hi Rick -- If I may, I think you're indicating the voltage on either side of the heater winding center tap at the power transformer. What I was hoping you could clarify is over at the output terminal strip -- the one labeled "Installation Connections". I show that terminal:

1 = 70 volts output.

1A = 20 volts output.

2 = Mystery Figure. It appears to be 3.something volts output, but I can't make it out to be sure. This is what I was hoping you could clarify for me.

2A = GND.

If you can, thanks in advance for checking!

Dave
 
Hi Rick -- If I may, I think you're indicating the voltage on either side of the heater winding center tap at the power transformer. What I was hoping you could clarify is over at the output terminal strip -- the one labeled "Installation Connections". I show that terminal:

1 = 70 volts output.

1A = 20 volts output.

2 = Mystery Figure. It appears to be 3.something volts output, but I can't make it out to be sure. This is what I was hoping you could clarify for me.

2A = GND.

If you can, thanks in advance for checking!

Dave


2 = 3.5V
 
Thanks so much!!

Dave
Hey Dave, "My information dates to June of 1980 when I came across one of these sans power transformer. My notes show I called Executone at the time, and through a discussion with one of it's representatives, was told that it was rated at 75 watts.
 
Hey Dave, "My information dates to June of 1980 when I came across one of these sans power transformer. My notes show I called Executone at the time, and through a discussion with one of it's representatives, was told that it was rated at 75 watts.

Sorry hit the post key... doh!... did you happen to ask back in 1980 what class type amplifier it was? Gadget73 is thinking it is a "PP AB1" as I'm leaning the same direction?
 
There is no doubt that based on the operating conditions and the design of the amplifier, it operates in Class AB1. Class A is simply not efficient enough for two KT88s to develop anywhere near the level of power output the unit is rated for, and the driver stage cannot deliver any power into the output tube grids which prevents positive grid drive. When these facts are coupled with the rather low level of standing current in the output stage, the only Class it can operate in is AB1.

Dave
 
Hi Dave,
I'm only getting back to work on this amp once again, so I like to just to confirm the output voltage tap to use for a direct to a speaker w/o the use of a 70V matching speaker transformer. This amp (if I done my math correctly) as you said is rated at 75 Watts with a speaker load of 4Ω, the voltage across speaker would equal, V=sqrt(75W*4Ω) = 17.32Vrms so then I could use a 4Ω speaker across the 20V output tap. So the difference in the tap voltage of 20V and calculated voltage of 17.32Vrms = 2.68V, this difference is not a concern to run the amplifier directly into a 4Ω speaker off of the 20V tap? I tried the math using a 8Ω and 16Ω speaker and the voltage is 24.94vrms and 34.64vrms no taps close to that.

Would possible a 6Ω speaker with a rating of 75W or more work best, as it is very close to the tap of 20V (75W*6Ω) = 21.21Vrms??

Question/s on a speaker for this application, is the rating in watts need to match 75W or greater to achieve max output power with only one speaker across the load?

If I wanted use more than just one speaker say total of 2 a pair of 8Ω speakers in parallel =4Ω (2 speaker @ 8Ω) will do the trick as well?
 
should be ok with a 4-8 ohm speaker on the 20 volt tap. Its not an absolutely perfect match, but close enough for gov't work.

Its more about reflected load on the primary. The amp will make best power when that matches properly, but its not going to kill it to run an 8 ohm load on what works out to be a 4 ohm tap. It just won't make as much power as it could.
 
new tubes working cropped.jpg

Update, sorry about the fuzzy pic, I purchased a set of new KT88 tubes after going through and checking for out of tolerance resistors and capacitors replaced all the power supply filter electrolytic capacitors, I adjusted the DC balance and adjusted the Bias circuit at 50mA (25ma each tube); I added a temporary input jack and speaker output jack and hooded a 4 ohm speaker cab directly to the amplifier (no matching transformer) and a guitar on the input and she is ALIVE :rockon:!!! No Hum, sound very clean, and musical to the ears:music:. Now that I have it running it needs a through cleaning:jump:
 
Nice Job! You would want each tube to draw 50 mA, for a total output stage quiescent current of 100 mA.

Dave
 
Nice Job! You would want each tube to draw 50 mA, for a total output stage quiescent current of 100 mA.

Dave
Dave I had though that as well about the bias current but when I checked them, the last tube set the bias pot was set to give me 24ma on one tube and 27ma on the other I adjusted the DC balance pot and they nearly squared off at 25ma each, the note on the schematic does say 50ma as you said I was thinking that was the total for both. Well I will later this week up the bias curent to 50am as you suggested and see what I have then as the volume of the amp at 25ma each tube was pretty loud can't wait to see what I have when I bump it up. I post back after adjusting and let you know. I been busy building a maggie 8603...
 

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At the proper bias setting, you will find the sound to be more dynamic and powerful. At 25mA per tube, power output will typically be reduced somewhat, and there will be significant crossover distortion.

Dave
 
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