Hey guys,
First time posting here although I've gotten some great info just perusing these threads. Anyhow, I recently scored an old bank intercom amp that is essentially 2 single ended amps in 1 with a common power transformer and 6x4 rectifier tube pushing 2 12AX7's and 2 6AQ5's with separate volume controls for each. I'm now trying to match up speaker impedance.
I found a filament transformer that puts out (yikes) 8.8 volts. I put this to the secondary side of one of the output transformers and got 93 volts on the primary side. (I applied the voltage backwards because I get a real low number stepping down from 8.8 volts that my meter won't read accurately). That equates to a 10.6:1 turns ratio or 112:1 impedance ratio. Using a 16 ohm load with this ratio I get 1792K ohms as a primary impedance. This does not even come close to matching the 5000K that a single ended 6AQ5 wants to see. It's even worse with 8 or 4 ohms. Just for laughs I hooked up an 8 ohm speaker to it and plugged in a guitar. The amp is quiet but the volume is low and sound is thin.
I used this same procedure on a known output transformer and got 7200K ohms primary impedance with an 8 ohm speaker load, so I think my calculation method is correct.
Any ideas or am I missing something?
Output Transformer Question
Moderators: pompeiisneaks, Colossal
Re: Output Transformer Question
The intercom speakers may have had a much higher impedance ?
Re: Output Transformer Question
I personally love the sound of a single ended 6AQ5 or 6BQ5. I had a 6AQ5 power amp rescued from a 1950s furniture-style record player/speakers/record storage unit. Made great sounds.
You're using the right approach: put in a voltage and see what voltage appears on the other winding. There are a couple of gotchas here that may be clouding things up for you.
First, it's possible that an intercom amp arrangement may not have used any modern speaker impedance. It's possible that they used 32 ohms or more, or that the output transformers drove "constant voltage" transformers for audio distribution. Both those would lead to odd results if you're looking to put a modern speaker on them.
It's also possible that the little trannies on the amps don't have enough low frequency response to make testing with 60Hz work out. It's possible that line frequency may be below the pass band and give you a falsely low output reading.
If you can do it, it would be much better to drive the speaker secondary of the OTs being tested with something like 1kHz from the speaker output of a separate amplifier and get whatever your meter can read well, probably just under 2V. That would get the frequency high enough to give an accurate reading of the ratios. It's a PITA, but you may have to.
I think you mean 5000 ohm (for example), not 5000K - that's 5 megohms.
You're using the right approach: put in a voltage and see what voltage appears on the other winding. There are a couple of gotchas here that may be clouding things up for you.
First, it's possible that an intercom amp arrangement may not have used any modern speaker impedance. It's possible that they used 32 ohms or more, or that the output transformers drove "constant voltage" transformers for audio distribution. Both those would lead to odd results if you're looking to put a modern speaker on them.
It's also possible that the little trannies on the amps don't have enough low frequency response to make testing with 60Hz work out. It's possible that line frequency may be below the pass band and give you a falsely low output reading.
If you can do it, it would be much better to drive the speaker secondary of the OTs being tested with something like 1kHz from the speaker output of a separate amplifier and get whatever your meter can read well, probably just under 2V. That would get the frequency high enough to give an accurate reading of the ratios. It's a PITA, but you may have to.
I think you mean 5000 ohm (for example), not 5000K - that's 5 megohms.
Re: Output Transformer Question
Yes, 100V or 70V speaker distribution systems are common in factory etc PAs http://en.wikipedia.org/wiki/Constant-v ... ker_system
Might 1kHz be above the max freq that many DMM sampling meters can accurately measure?
Might 1kHz be above the max freq that many DMM sampling meters can accurately measure?
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Re: Output Transformer Question
Thanks so much for the thoughtful replies. Oh yeah, definitely meant 5K ohms. It's probably easier to just get 2 new SE output trannies and be done with it. I kind of figured it was some sort of weird speaker arrangement but I like to reuse everything I can just cause I'm cheap. Tubes and More has 5K/8ohm 8 watt SE trannies for 13.95 each so that won't break the bank.
I want to keep the separate channels and run it into a stereo cab with the speakers angled out like 30 or 45 degrees. Haven't decided on 8 or 10 inch speakers yet. I think it will turn out to be a cool sounding and unique arrangement.
I want to keep the separate channels and run it into a stereo cab with the speakers angled out like 30 or 45 degrees. Haven't decided on 8 or 10 inch speakers yet. I think it will turn out to be a cool sounding and unique arrangement.
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gingertube
- Posts: 531
- Joined: Mon Nov 14, 2011 2:29 am
- Location: Adelaide, South Oz
Re: Output Transformer Question
With these amps there is another worthwhile rebuild option.
I just finished one - it had 70V line output trannies so I ended up basically keeping the power supply and stripping the rest. Then fitted a PP Output tranny (this one: http://www.mableaudio.com/en/productvie ... ormer.html) and turned it into a Tiny Terror clone
Schemo:
http://www.diyitalia.eu/forum/download/ ... &mode=view
Kept the 6X4 (Well 6CA4 in my case) rectifier power supply and the only other change from the TT schemo was the cathode bias resistor on the output tubes to suit other than EL84.
What was it like - well the little I used it, I liked it. That dual stage gain control allows easy adjust from clean to really mean. The local guitar god bought it of me on the spot the first time he tried it.
Cheers,
Ian
I just finished one - it had 70V line output trannies so I ended up basically keeping the power supply and stripping the rest. Then fitted a PP Output tranny (this one: http://www.mableaudio.com/en/productvie ... ormer.html) and turned it into a Tiny Terror clone
Schemo:
http://www.diyitalia.eu/forum/download/ ... &mode=view
Kept the 6X4 (Well 6CA4 in my case) rectifier power supply and the only other change from the TT schemo was the cathode bias resistor on the output tubes to suit other than EL84.
What was it like - well the little I used it, I liked it. That dual stage gain control allows easy adjust from clean to really mean. The local guitar god bought it of me on the spot the first time he tried it.
Cheers,
Ian
Re: Output Transformer Question
Gingertube,
Sounds like a cool build, especially the dual stage gain setup. Thanks for the schematic link. I'll file that one away for a future build. I wound up going with replacing the OT's and recapping the electrolytics. The amp sounds great, maybe a little dark but not pathologically so. True point-to-point, non-shielded input wiring and heater wires not twisted. The amp is dead quiet. This was the easiest build I've ever done. Not sure I can really call it a "build", more like a mod.
Sounds like a cool build, especially the dual stage gain setup. Thanks for the schematic link. I'll file that one away for a future build. I wound up going with replacing the OT's and recapping the electrolytics. The amp sounds great, maybe a little dark but not pathologically so. True point-to-point, non-shielded input wiring and heater wires not twisted. The amp is dead quiet. This was the easiest build I've ever done. Not sure I can really call it a "build", more like a mod.