Advice for building a SE tube amp in an old chassis

[Lavane]

Both the two Channel amp and mono blocks I have with the switches can be switched while on. Maybe read through everything on the Tubelabs site and see if it mentions it. I think you should be able to use it while on, but I would make sure first. You can email him and ask about it. He got back to me pretty fast with the question I had. Or send a PM over at DIY Audio.

 

[DrRick]

Where do you stand now with the completion of the amp? I’ve build hundreds of amps (mostly for clients, I’m an engineer) over the last 50+ years.
Perhaps you can mount the power xformer on it’s side and cut a perforated aluminum top with holes for the tubes to stick out?
Also, you could mount the toroidal output xformers on the bottom (underneath) of the chassis and use some sort of feet to elevate the chassis so you have .5-1 inch clearance between the outputs and the shelf the amp sits on. Or you could fabricate a skirt around the perimeter of the chassis to hide the outputs.
You can use the single meter to measure the plate current of each output tube by using the meter to indicate the voltage across each of the cathode resistors on the output tubes, using a switch to select which tube you want to measure. That would take a bit of simple ohm’s law calculations which I can help with. Many of the tube power amps used in movie theaters used this method as an in-circuit tube tester. The projectionist or tech could check the readings weekly and replace any output tubes that were losing emission. Those systems ran 12+ hours a day, seven days a week (or more) so the tech’s could replace tubes before the emissions dropped low enough to affect the sound quality.

Peace,
Rick

PS, was that a Dukane mixer you gutted? I started restoring/modifying one many years ago but lost some parts having moved twice since I started it. It’s on my “to be finished” list now that I’m retired. I may. Be interested in buying some of the left over parts from it if you care to sell them.

 

[Lavane]

Where do you stand now with the completion of the amp? I’ve build hundreds of amps (mostly for clients, I’m an engineer) over the last 50+ years.
Perhaps you can mount the power xformer on it’s side and cut a perforated aluminum top with holes for the tubes to stick out?
Also, you could mount the toroidal output xformers on the bottom (underneath) of the chassis and use some sort of feet to elevate the chassis so you have .5-1 inch clearance between the outputs and the shelf the amp sits on. Or you could fabricate a skirt around the perimeter of the chassis to hide the outputs.
You can use the single meter to measure the plate current of each output tube by using the meter to indicate the voltage across each of the cathode resistors on the output tubes, using a switch to select which tube you want to measure. That would take a bit of simple ohm’s law calculations which I can help with. Many of the tube power amps used in movie theaters used this method as an in-circuit tube tester. The projectionist or tech could check the readings weekly and replace any output tubes that were losing emission. Those systems ran 12+ hours a day, seven days a week (or more) so the tech’s could replace tubes before the emissions dropped low enough to affect the sound quality.

Peace,
Rick

PS, was that a Dukane mixer you gutted? I started restoring/modifying one many years ago but lost some parts having moved twice since I started it. It’s on my “to be finished” list now that I’m retired. I may. Be interested in buying some of the left over parts from it if you care to sell them.

Those are some good ideas. A perforated top would be a good idea if the power trany can be mounted on its side. Hole punches at Harbour Frt. are pretty cheap to punch holes in the perforated steel. Just drill a pilot hole for the punch and wrench it down to cut the holes for tubes. There is an old theater here in my town that has built in the early 1900's. It closed in the 80's, and never was really updated I side during its life. I would love to get I side that place and look for old audio equipment.

 

[Will2225]

Where do you stand now with the completion of the amp? I’ve build hundreds of amps (mostly for clients, I’m an engineer) over the last 50+ years.
Perhaps you can mount the power xformer on it’s side and cut a perforated aluminum top with holes for the tubes to stick out?
Also, you could mount the toroidal output xformers on the bottom (underneath) of the chassis and use some sort of feet to elevate the chassis so you have .5-1 inch clearance between the outputs and the shelf the amp sits on. Or you could fabricate a skirt around the perimeter of the chassis to hide the outputs.
You can use the single meter to measure the plate current of each output tube by using the meter to indicate the voltage across each of the cathode resistors on the output tubes, using a switch to select which tube you want to measure. That would take a bit of simple ohm’s law calculations which I can help with. Many of the tube power amps used in movie theaters used this method as an in-circuit tube tester. The projectionist or tech could check the readings weekly and replace any output tubes that were losing emission. Those systems ran 12+ hours a day, seven days a week (or more) so the tech’s could replace tubes before the emissions dropped low enough to affect the sound quality.

Peace,
Rick

PS, was that a Dukane mixer you gutted? I started restoring/modifying one many years ago but lost some parts having moved twice since I started it. It’s on my “to be finished” list now that I’m retired. I may. Be interested in buying some of the left over parts from it if you care to sell them.

I'm very happy with the amp now that it's built. I really love the sound and the feeling of building something like this is priceless.

I did end up mounting the output transformers on the bottom, and they don't extend below the bottom of the chassis. I put some feet on it too to allow for more ventilation. So far I've had no issues with running it for hours on end. The only time there was any issue was when using a different pre amp that I think caused an impedance mismatch and resulted in a fair amount of hum etc.

I did end up building a very cheap tube cage out of hardware cloth. I'll attach a photo or two here. The cage isn't perfect and could use some work but it's just for safety considering I have a cat.

I really would love to use the meter somehow in the build but as I'm an amateur I'd really need someone to walk me through it.

Considering it's a fixed bias amp, I'm not sure how helpful it would be to show the bias on the tubes for the meter, but perhaps I'm wrong. I'd be interested in hearing more ways to use the meter, whether it's for bias or to indicate a very rough measurement of volume. Or another option would be to modify the amp with different bias settings for different tubes, which could be interesting but maybe a bit beyond my skills.

Out of curiosity do you have any advice on how to hook up a headphone jack off the outputs? I've read a bit about it but being a beginner I'm too worried I would make a mistake to try it without help.

 

[Will2225]

Pictures of the not perfect tube cage.

 

[Lavane]

I'm very happy with the amp now that it's built. I really love the sound and the feeling of building something like this is priceless.

I did end up mounting the output transformers on the bottom, and they don't extend below the bottom of the chassis. I put some feet on it too to allow for more ventilation. So far I've had no issues with running it for hours on end. The only time there was any issue was when using a different pre amp that I think caused an impedance mismatch and resulted in a fair amount of hum etc.

I did end up building a very cheap tube cage out of hardware cloth. I'll attach a photo or two here. The cage isn't perfect and could use some work but it's just for safety considering I have a cat.

I really would love to use the meter somehow in the build but as I'm an amateur I'd really need someone to walk me through it.

Considering it's a fixed bias amp, I'm not sure how helpful it would be to show the bias on the tubes for the meter, but perhaps I'm wrong. I'd be interested in hearing more ways to use the meter, whether it's for bias or to indicate a very rough measurement of volume. Or another option would be to modify the amp with different bias settings for different tubes, which could be interesting but maybe a bit beyond my skills.

Out of curiosity do you have any advice on how to hook up a headphone jack off the outputs? I've read a bit about it but being a beginner I'm too worried I would make a mistake to try it without help.

Yes, you can. But you have to properly load the output transformers. You could build something with a switch, or you could build the circuit in a small project box and connect it to the speaker terminals. It you used banana jacks for speaker terminals it would be easy to unplug your speakers and plug the headphone into the speakers terminals. Here is a drawing showing how to do it using a 4ohm, or 8 ohm resistor depending on the taps your using. The 100 ohm resistor would go to the headphone jack.

 

[DrRick]

Your cage looks functional and ….organic?
I have cats of whom I am very protective of. They naturally avoid things (like tubes) that are physically hot, but they know even less about high voltage then I do, so nothing with “volts” is ever exposed in order to keep them safe (myself as well, I never thought it would happen but I’m getting older). I was taught when I interned at Bell Labs to always keep one hand in my pocket or behind me when working with live circuits. It’s amazing how little it takes across one’s chest to make one dead.

Lavane gave you a good start regarding adding a headphone output. The parallel resistor keeps a load on the output xformer and the series resistor drops the output level. It also drops the noice and/or hum level although using indirectly heated tubes that’s probably not an issue.

I’ll be back when I feel better to help you work through whatever you may need help with. This Covid thing is no joke! I’m ready for a nap from just the few words I typed in this post.
Peace,
Rick

PS: You’ve done some very nice work that you should be proud of!

I wonder why I’m noted as a “new member”/ I’ve been here for well over 15 years….

 

[Will2225]

I hope you're feeling better! I've been lucky and have yet to have covid so far, fully vaxed too but still you never know.

Also, thank you for your advice and kind words. My cat is the same, he stays clear of most of my audio equipment except the speakers and turntable, but I figure it's better to be safe than sorry.

I think I understand the diagram posted above but I just want to double check. I'm assuming the values of the resistors would be different in this situation but I would connect the 4ohm or 8ohm resistor across both the positive and minus wires connected to the headphone jack and then I would connect the 100 ohm to the positive of the headphone jack, correct? I'm a little confused on how the wiring works considering it's stereo and the signal is coming from two output transformers and not mono.

I need help in determining the correct resistor values to use for the headphone jack if I decide to go ahead with it. Also, what would be the best way to wire in a switch to defeat the speakers and switch to headphones? Apologies for being such a beginner when it comes to working on these things.

I'm also definitely still interested in figuring out how to get the meter working in this amp.

Once again, I hope you're feeling better and best of luck!

 

[DrRick]

I’ve not forgotten you, but I just feel awful!
I will get to you with further advise as soon as I can.
In the meantime, if you have any specific questions (especially of the “Yes or No” type, but not limited to that), fire away!

Peace,
The temporary broken DrRick

I hope you're feeling better! I've been lucky and have yet to have covid so far, fully vaxed too but still you never know.

Also, thank you for your advice and kind words. My cat is the same, he stays clear of most of my audio equipment except the speakers and turntable, but I figure it's better to be safe than sorry.

I think I understand the diagram posted above but I just want to double check. I'm assuming the values of the resistors would be different in this situation but I would connect the 4ohm or 8ohm resistor across both the positive and minus wires connected to the headphone jack and then I would connect the 100 ohm to the positive of the headphone jack, correct? I'm a little confused on how the wiring works considering it's stereo and the signal is coming from two output transformers and not mono.

I need help in determining the correct resistor values to use for the headphone jack if I decide to go ahead with it. Also, what would be the best way to wire in a switch to defeat the speakers and switch to headphones? Apologies for being such a beginner when it comes to working on these things.

I'm also definitely still interested in figuring out how to get the meter working in this amp.

Once again, I hope you're feeling better and best of luck!

 

[DrRick]

I’m finally starting to feel like myself, although I did about a year back and relapsed.
I’m sorry for not being more responsive.
Do you have any issues I can help with? I like to share what I’ve learned from 50+ years in audio engineering.

I may have that Ducane mixer chassis around. You could mount it on top of yours (without it’s top and bottom covers) and use a piece of perforated metal for the top.
You wouldn’t see much of the tubes ( I like having them exposed), but it would keep curious paws and fingers away from high voltage points and hot tubes.
You could also mount two meters on the front of it. I may have the Ducane VU meter that matches yours. Or there’s tons of VU Meter pairs with driver boards on ePay cheaply.
Peace,
Rick

 

[DrRick]

BTW,
The “dummy load” circuit looks ok, but if the headphones are low impedance they’ll serve as a load. The series resistor may be helpful in avoiding blasting your headphone diaphragms and your ears to hell.
Whatever you decide, you can experiment with the value of the series resistor to get sufficient range on the volume control. Also, if you use load resistors they needn’t be 50 watts. The amp probably doesn’t exceed 3-4 watts out so a 10 watt will run nice and cool. Typically I use non-inductive resistors in such an application but I doubt you’d notice any difference.
You can use a switching type headphone jack if you want to avoid a speaker switch. I used lots of 1/4” ones; I don’t know if 1/8” switching jacks are available.

(Less Sick) Rick