I need some help with the OPTs on a set of old 6V6GT monoblocks

[BoilermakerFan]

Does anyone have info on the old HICO OPTs and their part numbers?

I traded a set of headphones for a pair of quad 6V6GT monoblocks. I found out they were from a church sound system and were hooked up to the organ. Input tubes are 6SN7.

The part number on the OPT is AO-20927-1. HICO is still in business, but they only make large power transmission transformers now. I just need to know the impedance of the output taps. I have limited test equipment but I do have a 100MHz scope and DMM.

I have pics too if that would help.

Thanks.

[ChrisA]

Originally posted by BoilermakerFan
Does anyone have info on the old HICO OPTs and their part numbers?

I traded a set of headphones for a pair of quad 6V6GT monoblocks. I found out they were from a church sound system and were hooked up to the organ. Input tubes are 6SN7.

The part number on the OPT is AO-20927-1. HICO is still in business, but they only make large power transmission transformers now. I just need to know the impedance of the output taps. I have limited test equipment but I do have a 100MHz scope and DMM.

I have pics too if that would help.

Thanks.

You will need a 12 volt AC power cube. Put 12V on the primary and measure unloaded voltage on the seconday. Measure each tap

The voltage is proportional to the turns ratio. So you can figure out the ratio of primary to seconday turns.

Impedance is the square of the turns ratio. Once you know the ratios. Figure that one of the taps are for 4, 8 amd 16 ohms. you should be able to figure it out.

Actually 60Hz is a bit low for a test unless this is a very high quality transformer. the "standard test is done at 1Khz. If you have a very good audio signal generator use that. But the 12V power cube will actually make a very nice signal But only usable if the transformer is design to go below 60Hz. But likely it will.

A quicker way is to simply measure the DC resistance. the 16 ohm tap will have 2X the resistance of the 8 ohm to common tap and so on. But you need a good meter to measure low value resistance

[Wavebourn]

try edcorusa.com -- they made very good transformers for my tube amps.

Oops... Sorry, I thought you need a replacement.

I would try several standard options, loading it on 4,8,16 Ohm. Start from 16 Ohm, drive right before clipping. Then connect lower resistance in the row. If the lower one gives less than 90% of clean power increase it is too low. If more than 90% it is it.

[BoilermakerFan]

Thanks guys. I'll have to see if I have a 12V secondary transformer. Another friend suggested using my scope and a 1KHZ test tone from my PC sound card which would be 2V. I'm pretty sure they will go below 60Hz since this was the organ amp. Most churches I have been in have organs that go down to at least 40Hz and we have a church here in town that goes down to 18HZ.

Here are the pics:







I still need to trace everything out and draw up the schematic so I can see where upgrades make the most sense besides adding capacitance to the power supply.

[BoilermakerFan]

Originally Posted by ChrisA

You will need a 12 volt AC power cube. Put 12V on the primary and measure unloaded voltage on the seconday. Measure each tap

The voltage is proportional to the turns ratio. So you can figure out the ratio of primary to seconday turns.

Impedance is the square of the turns ratio. Once you know the ratios. Figure that one of the taps are for 4, 8 amd 16 ohms. you should be able to figure it out.

Actually 60Hz is a bit low for a test unless this is a very high quality transformer. the "standard test is done at 1Khz. If you have a very good audio signal generator use that. But the 12V power cube will actually make a very nice signal But only usable if the transformer is design to go below 60Hz. But likely it will.

A quicker way is to simply measure the DC resistance. the 16 ohm tap will have 2X the resistance of the 8 ohm to common tap and so on. But you need a good meter to measure low value resistance

Ah, back where it should be after the upgrade...

Chris,

Any reason why a 16Vac psu wouldn't work? I have a 16Vac PSU from an ol burglar alarm.

[aardvarkash10]

no reason at all - all you are looking for is a known safe working voltage AC. Proportionality is the main point.

Nice score btw - I hope they were seriously good headphones, otherwise the Devil is gonna have your soul!

[BoilermakerFan]

Originally Posted by aardvarkash10

no reason at all - all you are looking for is a known safe working voltage AC. Proportionality is the main point.

Nice score btw - I hope they were seriously good headphones, otherwise the Devil is gonna have your soul!

AT ESW9s, so yes, they were seriously good headphones. He's happy as hell with the headphones, I'm happy as hell with my monoblocks though they will have all the caps replaced and resistors tested, then add capacitance to the PSU. Conversion to RCA signal input, Neutrik PowerCon, and SpeakerCon connectors and the twins will be ready to drive a set of Fonkens or Castles.

I have about 5 other builds in front of them and two amps I need to build for Head-Fi's CanJam 2010, but they are a priority project for me. I need to get my hands on some nice figured wood for frames to hold the chassis after I have them stripped, painted, labeled, and cleared. The frames will be full dovetail corners though.

[ChrisA]

Originally Posted by BoilermakerFan

Ah, back where it should be after the upgrade...

Chris,

Any reason why a 16Vac psu wouldn't work? I have a 16Vac PSU from an ol burglar alarm.

The only reason I said "12V" is so that you have a safe voltage. 16V is fine.

But still you are testing impedance at 60Hz (or 50Hz in some places) and this may be different from what you'd get if you have a 1KHz test signal which is the "standard" used for impedance measurement. Likely "close enough"

If you need a 1Khz test signal it is pretty easy to get if you have a computer with an audio interface (sound card). Lacking that get a CD with test signals and play i through an old walkman. Put 60Hz is maybe close enough and easy to do.

[BoilermakerFan]

Thanks Chris.

[ChrisA]

Originally Posted by BoilermakerFan

Thanks guys. I'll have to see if I have a 12V secondary transformer. Another friend suggested using my scope and a 1KHZ test tone from my PC sound card which would be 2V. I'm pretty sure they will go below 60Hz since this was the organ amp. Most churches I have been in have organs that go down to at least 40Hz and we have a church here in town that goes down to 18HZ...... so I can see where upgrades make the most sense besides adding capacitance to the power supply.

Two points:

(1) I have a Hammond amp that looks a lot like yours. I think mine is an older version. If so the schematic is available on-line. If I'm right then the amp has three channels (not stereo but two treble and one bass. On the Hammond organs the bass amp drove 15" bass speakers and the each of the others channels drove treble speakers, one of these had reverb added. So your assumption is only half correct. One of the channels is designed for low frequency the other two are not. Today HiFi people use the term "bi-amp" but Hammond went one step more and actually custom built the bass and treble amps. Also my amp was a "balanced" design, not at all like a typical guitar or hifi amp. These amps were fed a balanced input and were push pull but with no need for a phase inverter.

(2) Before you add capacitance check the data sheet for the rectifier tube. Many times the capacitance needs to be low so as to reduce in-rush current when the AC mains power is switched on. Simply adding a big cap without doing the math can blow the rectifier tube. I'm not saying "don't do it", just be sure and think about in-rush current limits, or convert to solid state where the limits are hard to exceed.

You might be able to find the schematic here (maybe some version of HR40?)
http://www.captain-foldback.com/Hamm...schematics.htm
Even if your amp is not here Hammond re-used the same design ideas. For example All my OPTs have the "extra leads" that I could not figure out until I looked at the schematics. It seams that there is a secondary winding on each OPT just for negative feedback. The reason (I think) for this is that tHammond's balanced amp needs two negative feedback signals 180 degrees out of phace so they use a center taped winding just for this purpose. The more usual design of tapping the speaker output could not work.