Help designing a tube amp...

.... · 5th February 2006, 08:44 AM

This is my first post on your forum, I hope I am not breaking too many rules

OK, I want to build a tube amp. I really just want to do it for the heck of it, and to get the learning experience out of it. I will admit I am a t00b n00b, although I do have a general idea as to how they work and I have used them before (in a vacuum tube Tesla coil ). I also am knowledgeable in electronics in general and am experienced in working with high voltage, please do not try to oversimplify things for me.

That out of the way, the main goal of this project is to produce a working stereo tube amp (it can use semiconductors, but I want it to be tube based) that can use a normal line (1v p-p) level input, and can fill a 20'x20' room with sound at a decent volume (about what 50w of transistor power would give). I want to keep the price decently low (would $50 be reasonable with a well stocked junk box and some scrounging?), that sounds ok (as good as a decent transistor amp).

I have on hand that I would like to use...
-some car speakers I would like to use for the speakers, but realise they are probably not the ideal speaker.

-some tubes; a pair of used 6080 dual triodes and a pair of 6146 single triodes I would like to use (to keep cost down), would they be reasonable for the power out I want?

-assorted caps/resistors. I think I have most everything I will need short of the magnetics and possibly the tube

As far as what I need to get, I was thinking this for the output transformer. Then as far as the b+/filament transformer I was thinking of ebaying one of the right voltage (need to decide on a tube first).

Thanks!

Shoog · 5th February 2006, 09:34 AM

Car driver in TWQT pipes can sound amazingly good. They do have a tendency to brightness. I used them at the start. They have the advantage of been very sensitive and fairly robust. Consider having the drivers mounted firing at the ceiling or at 45 degrees to the front. Also dammaring the cones can help a lot. Not all car drivers are made equal though. Older Pioneers can sound excellent. I have a pair of coxials mounted in the wall behind my head as I write.

You will need to consider paralleling your 6080's to even get 4W of SE power. You might be better considering PP for a little more power. If going SE consider going parafeed. You could get by with a +B of only 150V if you used simple mosfet CCS's. The advantage of parafeed is that you could use toroidal mains transformers as output iron. These will perform much better than your suggested airgapped iron (designed for guitar means designed to distort). Output blocking caps shouldn't be an issue at these voltages, you could even use bypassed electroytics initially. Doing it this way also cuts down on power supply costs as CCS loading has excellent PSRR. You might just bring your project in at you suggested budget.

Shoog

Eli Duttman · 5th February 2006, 02:14 PM

Consider the "El Cheapo" project. While not $50, it is very low budget and at approx. 8 WPC a realistic choice for some listening volume. Look here.

BTW, you and I are thinking similarly. "El Cheapo" uses this T/E O/P trafo.

.... · 5th February 2006, 04:49 PM

This is looking promising...

As far as el cheapo, I noticed here there is a kit available for a little over $100 (I could handle that) but it seems to be lacking the output iron (another $50)... What exactly would I have to obtain to build el cheapo?
I could probably handle $150 if I knew I was going to have all of the right parts, and it would work when I was done...

But back to building something with my tubes....
First, how much 'tube' power will it take? I believe that the sound output for a tube amp at a given o/p power is higher than a transistor amp because they only amplify the audible range (or something like that) Second, will the tubes I have now be able to do the job? I would like to know early on if they will work so I know if I should be trying to build a design around them, or if I am starting from scratch...

Also, what do se, psrr, and parafeed mean?

Thanks!

kevinkr · 5th February 2006, 05:32 PM

PP 6080 might give you up to 10Wrms with careful design, you would be better off selling the 6146's on eBay and adding whatever money you get for them to the pot.

IMHO You might be better off finding a clapped out scott, fisher, radio shack, lafayette or dyna amplifier using 6BQ5/7189 and salvaging them for the iron, and perhaps even the chassis. If you are lucky you will end up with a set of usable tubes as well. This will also put you in the range of 15w pc which is reasonable for the speakers you proposed.

Kevin

.... · 5th February 2006, 06:18 PM

hmm, how would I find a dead amp that has the needed magnetics? I can't just type 'dead amp based on 6bq5' into ebay... Is there a specific model or something I should look for?
Thanks!

Shoog · 5th February 2006, 09:06 PM

Do a search on parafeed, there are a few designs on the net - it will explain the basic principle. PSRR means Power Supply Rejection Ratio and referes to the ability of the circuit to reject power supply hum from the output. SE(single ended) designs have poor PSRR and so need more power supply filtering and hence expense, Push Pull designs have good PSRR and Parafeed amps have the best PSRR and so cheap power supplies.

Your not going to achieve you objective of designing a usable power amp without knowing the basic principles and terminology. A bit of time spent researching the field of tubes would help a lot. The are plenty of basic resources out on the net.

Shoog

.... · 6th February 2006, 07:45 AM

sorry about the acronyms, now that they have been defied they seem stupidly obvious, the forum I usually live on just uses different ones.

Hmm, so in a parafeed design you are using the voltage drop across a separate inductor (which is stepped down by an xformer to match the independence of the speaker) as opposed to using the inductance of the o/p xformer? I an see how this would be beneficial...

Shoog · 6th February 2006, 08:51 AM

Suitable inductors are hidiously expensive - so forget about that as an option. A simple solid state CCS does a better job and eliminates possable resonante issues. The inductor or the CCS provides a constant DC current - which is usually done by the output transformer in an SE design. It also acts as the output load in parallel with the output transformer. Since a CCS has a very high output impedance at AC, it actually contributes very little to the actual AC load which the anode see's. The anode load is dominated by the much lower impedance of the output transformer.

Shoog

Original Burnedfingers · 6th February 2006, 10:39 AM

Considering what you have to spend I would recommend shopping on ebay for some used organ amplifiers. I have seen working Wurlitzer 6L6 PP amps going for atound $25 ea with tubes. Not a bad place to start.

5th February 2006, 08:44 AM	#1
.... diyAudio Member Join Date: Feb 2006 Location: Arcadia, USA	Help designing a tube amp... This is my first post on your forum, I hope I am not breaking too many rules OK, I want to build a tube amp. I really just want to do it for the heck of it, and to get the learning experience out of it. I will admit I am a t00b n00b, although I do have a general idea as to how they work and I have used them before (in a vacuum tube Tesla coil ). I also am knowledgeable in electronics in general and am experienced in working with high voltage, please do not try to oversimplify things for me. That out of the way, the main goal of this project is to produce a working stereo tube amp (it can use semiconductors, but I want it to be tube based) that can use a normal line (1v p-p) level input, and can fill a 20'x20' room with sound at a decent volume (about what 50w of transistor power would give). I want to keep the price decently low (would $50 be reasonable with a well stocked junk box and some scrounging?), that sounds ok (as good as a decent transistor amp). I have on hand that I would like to use... -some car speakers I would like to use for the speakers, but realise they are probably not the ideal speaker. -some tubes; a pair of used 6080 dual triodes and a pair of 6146 single triodes I would like to use (to keep cost down), would they be reasonable for the power out I want? -assorted caps/resistors. I think I have most everything I will need short of the magnetics and possibly the tube As far as what I need to get, I was thinking this for the output transformer. Then as far as the b+/filament transformer I was thinking of ebaying one of the right voltage (need to decide on a tube first). Thanks!

5th February 2006, 02:14 PM	#3
Eli Duttman diyAudio Member Join Date: Apr 2004 Location: Plainsboro, NJ	Consider the "El Cheapo" project. While not $50, it is very low budget and at approx. 8 WPC a realistic choice for some listening volume. Look here. BTW, you and I are thinking similarly. "El Cheapo" uses this T/E O/P trafo. __________________ Eli D.

5th February 2006, 05:32 PM	#5
kevinkr diyAudio Moderator Join Date: Sep 2004 Location: Boston, Massachusetts Blog Entries: 6	PP 6080 might give you up to 10Wrms with careful design, you would be better off selling the 6146's on eBay and adding whatever money you get for them to the pot. IMHO You might be better off finding a clapped out scott, fisher, radio shack, lafayette or dyna amplifier using 6BQ5/7189 and salvaging them for the iron, and perhaps even the chassis. If you are lucky you will end up with a set of usable tubes as well. This will also put you in the range of 15w pc which is reasonable for the speakers you proposed. Kevin __________________ www.kta-hifi.net

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5th February 2006, 09:34 AM	#2
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Car driver in TWQT pipes can sound amazingly good. They do have a tendency to brightness. I used them at the start. They have the advantage of been very sensitive and fairly robust. Consider having the drivers mounted firing at the ceiling or at 45 degrees to the front. Also dammaring the cones can help a lot. Not all car drivers are made equal though. Older Pioneers can sound excellent. I have a pair of coxials mounted in the wall behind my head as I write. You will need to consider paralleling your 6080's to even get 4W of SE power. You might be better considering PP for a little more power. If going SE consider going parafeed. You could get by with a +B of only 150V if you used simple mosfet CCS's. The advantage of parafeed is that you could use toroidal mains transformers as output iron. These will perform much better than your suggested airgapped iron (designed for guitar means designed to distort). Output blocking caps shouldn't be an issue at these voltages, you could even use bypassed electroytics initially. Doing it this way also cuts down on power supply costs as CCS loading has excellent PSRR. You might just bring your project in at you suggested budget. Shoog

5th February 2006, 04:49 PM	#4
.... diyAudio Member Join Date: Feb 2006 Location: Arcadia, USA	This is looking promising... As far as el cheapo, I noticed here there is a kit available for a little over $100 (I could handle that) but it seems to be lacking the output iron (another $50)... What exactly would I have to obtain to build el cheapo? I could probably handle $150 if I knew I was going to have all of the right parts, and it would work when I was done... But back to building something with my tubes.... First, how much 'tube' power will it take? I believe that the sound output for a tube amp at a given o/p power is higher than a transistor amp because they only amplify the audible range (or something like that) Second, will the tubes I have now be able to do the job? I would like to know early on if they will work so I know if I should be trying to build a design around them, or if I am starting from scratch... Also, what do se, psrr, and parafeed mean? Thanks!

5th February 2006, 06:18 PM	#6
.... diyAudio Member Join Date: Feb 2006 Location: Arcadia, USA	hmm, how would I find a dead amp that has the needed magnetics? I can't just type 'dead amp based on 6bq5' into ebay... Is there a specific model or something I should look for? Thanks!

5th February 2006, 09:06 PM	#7
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Do a search on parafeed, there are a few designs on the net - it will explain the basic principle. PSRR means Power Supply Rejection Ratio and referes to the ability of the circuit to reject power supply hum from the output. SE(single ended) designs have poor PSRR and so need more power supply filtering and hence expense, Push Pull designs have good PSRR and Parafeed amps have the best PSRR and so cheap power supplies. Your not going to achieve you objective of designing a usable power amp without knowing the basic principles and terminology. A bit of time spent researching the field of tubes would help a lot. The are plenty of basic resources out on the net. Shoog

6th February 2006, 07:45 AM	#8
.... diyAudio Member Join Date: Feb 2006 Location: Arcadia, USA	sorry about the acronyms, now that they have been defied they seem stupidly obvious, the forum I usually live on just uses different ones. Hmm, so in a parafeed design you are using the voltage drop across a separate inductor (which is stepped down by an xformer to match the independence of the speaker) as opposed to using the inductance of the o/p xformer? I an see how this would be beneficial...

6th February 2006, 08:51 AM	#9
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Suitable inductors are hidiously expensive - so forget about that as an option. A simple solid state CCS does a better job and eliminates possable resonante issues. The inductor or the CCS provides a constant DC current - which is usually done by the output transformer in an SE design. It also acts as the output load in parallel with the output transformer. Since a CCS has a very high output impedance at AC, it actually contributes very little to the actual AC load which the anode see's. The anode load is dominated by the much lower impedance of the output transformer. Shoog

6th February 2006, 10:39 AM	#10
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Considering what you have to spend I would recommend shopping on ebay for some used organ amplifiers. I have seen working Wurlitzer 6L6 PP amps going for atound $25 ea with tubes. Not a bad place to start.

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