Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on April 22, 2013, 05:41:59 pm
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Looks like I got this all wrong. Check my next post..... :BangHead:
This "looks" like about what you would see in a 20-30 Watt amp.
But it is marked (I will measure and confirm) 2.5k. That is what I would think is an "ideal" primary for a quad of 6L6.....a 100 watt amp.
I do not know the history of the transformer. Assuming it is a 2.5k primary.....what amp would you guys build for an OT of this size and impedance.? What power tubes, and how many would be appropriate.?
Thank You
Forgot this has two taps.
I did some measuring.
Looks like I can do:
16 Ohms at 2.5k
4 Ohms at 7.5k
http://s111.photobucket.com/user/zzmoore/library/OT%20quest (http://s111.photobucket.com/user/zzmoore/library/OT%20quest)
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with the 18ga on the secondaries, i doubt it's an inter-stage.
7.5K:4 - 2x6V6 or 2xEL84.
--pete
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Not sure what I was seeing before, it is different now. I double checked and this is what I get:
Primary 11.8 VAC
Secondary 0.34 VAC
That gives me 11.8/0.34 =34.8 ...correct.?
34.8 x 34.8 = 1,211
That number is on Both secondaries. Green to Yellow, and Green to Black. VERY little difference.
So if I am doing this right...
That is:
2 Ohms at 2.5k
4 Ohms at 4.8k
8 Ohms at 9.7k
Assuming I have done this properly.....it is kind of a strange Out Tranny.?
Thank You
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So if I am doing this right...
That is:
...
8 Ohms at 9.7k
Measure again. Assume the black is probably the common lead for the secondary, and that the taps are for 8Ω (black to lower voltage, probably yellow, tap) and 16Ω (black to higher voltage, probably green, tap).
Some research is needed. If you google the numbers on the transformer, you eventually find out this is a transformer from a Guild New Englander (http://www.diytube.com/phpBB2/viewtopic.php?t=4550) console stereo (http://www.radiomuseum.org/r/guild_the_new_englander_stereo.html).
This beast had 2 push-pull EL84 output stages for the left and right channels, plus a clever way to add a center bass channel that was mono. Basically, this transformer has 2x EL84's on either end of the primary or 4 total EL84's in parallel-push-pull. Schematic here (http://www.alanp.com/pics/Guild2.pdf).
Before you ask:
The left and right channels are push-pull. However, the mono bass channel also has its own phase inverter. Each output of the bass phase inverter is applied common mode to one of the left/right channel's output tubes. The left/right channels have smaller OT's just for highs, while the common-mode signals for the bass channel get applied only to its output transformer (because the small OT's reject the common-mode signal but still pass that current).
So the OT is suitable for 4x EL84's in push-pull, but may lack treble (or not, depending on its quality).
Schematics were found here (https://groups.google.com/forum/?fromgroups=#!topic/rec.antiques.radio+phono/Oy9z5SDaECo).
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Interesting.....
Give me time to absorb this. :think1:
In the meantime I will measure again.
Thank You So Much
I Appreciate It
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Interesting.....
Give me time to absorb this. :think1:
In the meantime I will measure again.
Thank You So Much
I Appreciate It
kindly measure both secondaries?
--pete
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OK -
Now using Black as the common, and 11.8 VAC at the primary:
Yellow wire is 0.663.
Green wire is 0.937
Assuming a reflection of 2.5k Ohms.....it looks like:
Yellow 8 Ohms
Green 16 Ohms
Am I finally getting a rope around these numbers.?
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OK -
Now using Black as the common, and 11.8 VAC at the primary:
Yellow wire is 0.663.
Green wire is 0.937
Assuming a reflection of 2.5k Ohms.....it looks like:
Yellow 8 Ohms
Green 16 Ohms
Am I finally getting a rope around these numbers.?
affirmative. :-)
--pete
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The architecture of the amp HotBluePlates posted the schematic is very interesting
But very far from my brain comprehension abilities :bump1:
K
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Assuming a reflection of 2.5k Ohms.....it looks like:
Yellow 8 Ohms
Green 16 Ohms
Don't assume anything for a primary. Assume a secondary speaker load, because that's what you have control of (the impedance of the speakers you connect).
You also didn't have to assume the primary, because the 8 and 16Ω impedances are indicated in the hookup diagrams on the schematic. That allows you to calculate the primary impedance to know its value for certain.
Now using Black as the common, and 11.8 VAC at the primary:
Yellow wire is 0.663.
Green wire is 0.937
If it were me, I'd connect the known a.c. voltage to the secondary, and measure high voltage a.c. on the primary. Obviously, hook the meter leads up with clamps before applying power.
Why? You're getting small quantities for the secondary voltage. Maybe your meter is accurate there, maybe not. Accuracy and precision is less of a problem when measuring high voltages.
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The architecture of the amp HotBluePlates posted the schematic is very interesting
But very far from my brain comprehension abilities :bump1:
As far as the Bass channel is concerned, there is only a wire from the output of either end of its phase inverter to the EL84 grids, and from the EL84 plates to the Bass channel OT.
The Left channel doesn't see the Bass channel input, because it is applied equally to the push- and pull-sides of the Left channel output transformer. Therefore, none of the Bass channel content passes to the Left channel OT secondary; however, the current due to the bass channel input is still drawn through the Left channel EL84s and through the Left channel OT primary.
The same thing is happening in the Right channel OT.
The signal was divided among Left, Right and Bass channels in the preamp of this console.
Maybe I missed it, but HBP has provided you with the basis for your amp.
I implied it, but here's me saying it explicitly:
Build something like an AC-30; 4x EL84's in push-pull. That is exactly how this OT was used in the original device it has been scavenged from.
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1. Are you saying to turn the OT around, and put the 11.8 VAC across the secondary., and then measure the Primary.? That will give a much higher VAC for my meter. Will give it a try.
2. Build an AC30...Aren't there 2 of these OT running the 4xEL84 power amp.? I thought that One OT would run 2xEL84 or 2x6V6.
Thank You
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1. Are you saying to turn the OT around, and put the 11.8 VAC across the secondary., and then measure the Primary.? That will give a much higher VAC for my meter. Will give it a try.
You don't have to do it this time. But yes, applying say 5-10vac (or whatever you have available) to the secondary will generate a high voltage on the primary. What I'm getting at is your meter (all meters, really) will be more accurate when measuring a high voltage than when measuring a low voltage.
A special case is if you're using an analog meter: now your meter will be most accurate somewhere around 1/2 to 3/4 of full-scale. You'd generally select the range setting to land in that area when applying a voltage whose likely value you can reasonably guess. If you don't know what voltage to expect, you start on the highest voltage range of your meter, and gradually switch to lower ranges to get the needle to land 1/2-3/4 of full scale.
2. Build an AC30...Aren't there 2 of these OT running the 4xEL84 power amp.? I thought that One OT would run 2xEL84 or 2x6V6.
Did you pull this transformer yourself from an old chassis? If no, did you look at the Guild New Englander link I posted earlier?
This stereo has 3 output transformers: Left channel (for mids-highs), Right channel (for mids-highs), Bass channel (mono, for bass from both left and right channels).
The Left and Right OT's are driven by 2x push-pull EL84's each. The Bass channel OT has a clever arrangement to be driven by the Left channel's EL84's and the Right channel's EL84's. It acts as though the Left channel are 2x parallel EL84's and the Right channel is another 2x parallel EL84's; the 4x total EL84's drive the Bass OT in push-pull, like an AC-30.
Your next question should be, "But doesn't an AC-30 use an OT that's about 4kΩ?"
Yes, the AC-30 does. The Guild console stereo ran the EL84's at 275v on the plate, enabling the lower OT primary impedance for the same output power.
If you have to run a higher B+, more like the AC-30's 320v, you could put 16Ω of speaker load on the 8Ω tap to reflect a 5kΩ primary impedance.
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Your next question should be, "But doesn't an AC-30 use an OT that's about 4kΩ?"
Yeah That is exactly what I was going to ask... :laugh:
So that settles that un-asked question.
FWIW...I went ahead and flipped the voltage source around...just to confirm. As you can imagine, the numbers were just a bit different, but it did verify I have it right.
I will continue that in the future.
To answer you about where the OT came from.....I bought them from a guy who said that HE pulled them from an Organ, but I do not know which model.
Thank You For Your Help.
Thanks
btw.....my little 12.8 VAC tranny died. So I used a Class-2 "wall wart" that measures 13.8 VAC. As I say, the numbers were very close. So again, I ASSUME there is no "problem" with using this wall wart for Transformer testing like this.?
Thank You
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btw.....my little 12.8 VAC tranny died. So I used a Class-2 "wall wart" that measures 13.8 VAC. As I say, the numbers were very close. So again, I ASSUME there is no "problem" with using this wall wart for Transformer testing like this.?
I don't see a problem, as long as it's a.c. output. You only need a source of a.c. voltage whose value you can measure. It's helpful to not be dealing with 120vac from the wall for such testing.
You could use a lower voltage like 5vac, since as you noted, with the a.c. applied to the secondary the primary output voltage will be high.
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At the moment, that is the only source of low voltage AC that I have. I guess I could Hot-Up a spare power tranny and use the 5/6 Volt heater supply, but that seems more trouble than it is worth.
Thanks Again
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@ HotBluePlates
The Left channel doesn't see the Bass channel input, because it is applied equally to the push- and pull-sides of the Left channel output transformer. Therefore, none of the Bass channel content passes to the Left channel OT secondary; however, the current due to the bass channel input is still drawn through the Left channel EL84s and through the Left channel OT primary.
The same thing is happening in the Right channel OT.
I've read your post yesterday in the morning, but I was still a bit confused
now here is 04.31 AM and I've read your new post
The Left and Right OT's are driven by 2x push-pull EL84's each. The Bass channel OT has a clever arrangement to be driven by the Left channel's EL84's and the Right channel's EL84's. It acts as though the Left channel are 2x parallel EL84's and the Right channel is another 2x parallel EL84's; the 4x total EL84's drive the Bass OT in push-pull, like an AC-30.
Many Many thanks I've understand how it works !
Grazie
Franco