Hello.
I have a question about a half dozen pairs of output transformers that I have on hand. Two pairs came out of console amp chassis that I got at an auction years ago, and the four other pairs were purchased from a gent who used to repair radios and TVs and was cleaning house. In the case of the console amp pulls, if I recall correctly, one pair was from an amp using ECL84s, and the other pair was from an amp using EL84s. The sources for the other four pairs are unknown, but they are probably from consoles as well, or stereo tabletop radios.
All of these transformers measure at from 10-12K to 8 Ohms, with 11K to 8 Ohms being the average. They are sized like a typical 4-6 watt output transformer. This turns ratio seems high, especially considering their source being definitely console amps in the case of two of them, and probably console amps for the other four.
I would like to build a bunch of triode connected pentode amps or pentode connected amps, using these OPTs, and give them away to friends and family. I have a bunch of different tubes that I can use with these OPTs, but mostly I would like to use some of the many 6P3S tubes (not 6P3S-e) that I have on hand.
I have two questions. If I use the 6P3S tubes, which are a 19 Watt max dissipation Russian analog of the 6L6G, will the high primary of 10-12K for these OPTs be a problem? The second question is if the 6P3S tubes are a bad match, what would be an appropriate tube for these OPTs that is very reasonably priced? Most of the power transformers that I have on hand will put out 250 to 300 volts of rectified DC. Thanks in advance for your replies!
I have a question about a half dozen pairs of output transformers that I have on hand. Two pairs came out of console amp chassis that I got at an auction years ago, and the four other pairs were purchased from a gent who used to repair radios and TVs and was cleaning house. In the case of the console amp pulls, if I recall correctly, one pair was from an amp using ECL84s, and the other pair was from an amp using EL84s. The sources for the other four pairs are unknown, but they are probably from consoles as well, or stereo tabletop radios.
All of these transformers measure at from 10-12K to 8 Ohms, with 11K to 8 Ohms being the average. They are sized like a typical 4-6 watt output transformer. This turns ratio seems high, especially considering their source being definitely console amps in the case of two of them, and probably console amps for the other four.
I would like to build a bunch of triode connected pentode amps or pentode connected amps, using these OPTs, and give them away to friends and family. I have a bunch of different tubes that I can use with these OPTs, but mostly I would like to use some of the many 6P3S tubes (not 6P3S-e) that I have on hand.
I have two questions. If I use the 6P3S tubes, which are a 19 Watt max dissipation Russian analog of the 6L6G, will the high primary of 10-12K for these OPTs be a problem? The second question is if the 6P3S tubes are a bad match, what would be an appropriate tube for these OPTs that is very reasonably priced? Most of the power transformers that I have on hand will put out 250 to 300 volts of rectified DC. Thanks in advance for your replies!
The OPTs don’t appear to be that old. They seem much newer than the single OPTs that I have from old 4 or 5 pin tube radios, and all appear to be used, with shorter, cut leads. There being two of each makes me think that the four pairs from the radio repair gent were from stereo sets. Maybe the sets had two low Ohm speakers per channel?
DF96: Thanks for the info. That would explain the high ratio of the transformers. Using these transformers with an 8 Ohm speaker will reduce output, but also reduce distortion as well, if I understand it right. I’ll do some load lines and see what the output will be at the voltages I expect. Are there any other repercussions associated to using a higher than specified transformer for a given tube? Most of the transformers on hand are at least double the ratio recommended for the tubes I want to use.
Most tube radios and TV sets had a 3W or 5W transformer for 3 ohm speakers and 40-50 mA anode current. Frequency response over 10 KHz sometimes is good, often is not - you should check. Electrostatic tweeters were used to overcome this shortcoming. With today 8 ohm speakers, the reflected primary impedence becomes 10-11 Kohm and a good match could be PL95 or 6AK6. I tried both and they work fine, if the transformer has a good enough quality. Direct heated output tubes for dry cell battery radios such as DL96 are another option.
I don't see where you say if these are SE or PP.
5K-6K:4r would be a super-typical SE OT. 265V supply will give ~~5 Watts out, 190V for 3.5W out. Assuming 40% efficiency, you need a Pdiss of 9 to 12 Watts. You certainly can run a 19W tube at part-power, and gain life, but the heater is probably hungrier than a 12W tube.
5K-6K:4r would be a super-typical SE OT. 265V supply will give ~~5 Watts out, 190V for 3.5W out. Assuming 40% efficiency, you need a Pdiss of 9 to 12 Watts. You certainly can run a 19W tube at part-power, and gain life, but the heater is probably hungrier than a 12W tube.
Hi PRR. They are all single ended. It's in the title. They are mostly around 7K:4 Ohm, so just over the 5K-6K:4 Ohms that you refer to in your post. I have many old power transformers that I can use, so anything from 150-350 volts rectified is available as a B+. I am assuming that you are referring to connecting the amps as pentodes when you refer to 40% efficiency, and that a pentode connection, and a lower dissipation than the 19 Watts possible, is the way to go if I use the 6P3Ss that I have on hand with these OPTs. Is that correct?
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It is likely that these OPTs are not of the highest quality so don't expect wide frequency response, especially at the bass end. Using them at a higher impedance will reduce the bass still further. If you can, measure the primary inductance.
The HF resonance may be low too, so you may have to be careful if you intend using feedback.
The HF resonance may be low too, so you may have to be careful if you intend using feedback.
I have a DMM that reads inductance up to 20H. I tried it across the primaries, with the secondaries not connected to each other. On the three sets I tested, the primary inductance was not identical between the two OPTs in each pair, but close. The three sets measured around 6H, 7H and 10H. As a reference, I measured a 3 watt 5K:8 Ohm single ended OPT made by Electraprint that I have on hand, and it measures 7.5H.
> measured around 6H
Assuming 5K:4r, that gives a 130Hz low-cut.
This is quite in line with stiff 8" speaker in open-back cabinet. I think ~5K *is* the primary nominal rating.
Significant NFB can extend the small-signal bandwidth to <50Hz, but it's gonna be dirty with high-level rock-n-roll.
With triode drive you may do 3X better. Which is not shabby.
Assuming 5K:4r, that gives a 130Hz low-cut.
This is quite in line with stiff 8" speaker in open-back cabinet. I think ~5K *is* the primary nominal rating.
Significant NFB can extend the small-signal bandwidth to <50Hz, but it's gonna be dirty with high-level rock-n-roll.
With triode drive you may do 3X better. Which is not shabby.
Because it halves the primary inductance and doubles the low end corner frequency (-3 dB).
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