• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Car Tube Amp

Craig D

Member
2007-12-25 6:03 pm
Hi all. This is my first post. I'm also quite new to tube amps but really like what I hear. I am busy building myself a tube amp for my car. As i'm quite inexperienced I've taken advise from a friend to follow the simple circuit diagram of the Dynaco ST-70. All is going well and I have all the parts including the chassis. Where I am feeling a bit concerned is in the preamp/phase splitter tube. The original circuit requests a 7199 that is totally unavailable in my part of the world. With a slight mod I could use the 6U8. When I google the 6U8 I seem to find very negative opinions on it. Could someone advise me on whether this tube is in fact ok or if there is another tube or even 2 tubes (one for the preamp and one for the phase stlitting) that I could use instead. If so possible diagram on connecting up would be appreciated. Thanks. Craig.
 
In the ham radio world, a lot of people complain of short tube life in 6U8, mostly from grid to cathode shorts. I've never had a 6U8 go bad, and I have a lot of them in service in various radios.

6EA8 and 6GH8 are common replacements. I have no idea how any of them would work / sound in audio service.

Win W5JAG
 
6AN8 works very well in the similar circuit of the Dynaco Mk-II with P-P UL 6CA7`s.

For the vibration in a car amp I think I`d be looking at more robust output tubes like 6146 or 829B. Float the output tubes on a subchassis plate on rubber isolator suspension.

If you want to be really classy get a vintage dynamotor in the trunk to provide the B+ supply. :D
 

7N7

Member
2003-01-18 10:43 pm
England
ray_moth said:
I wouldn't call a 6146 particularly robust. 807 would be a better bet, IMHO.



That sounds like a good description of what killed off British Leyland. :D

British Leyland was killed before birth, by first, the stupid incompetent Labour government that conceived the idea (it should never have existed) and second, by that ****** Stokes who ran it. End of story.

7N7

* I am not allowed by this forum to type the word w****r. I hope that you know what w****r means to English eyes.
 
ray_moth said:
I wouldn't call a 6146 particularly robust. 807 would be a better bet, IMHO.



That sounds like a good description of what killed off British Leyland. :D

The 6146 got banged about in many mobile radiotelephones and taxi radios on the 2 meter band (where one tube could make 30 watts RMS FM carrier) and seemed to survive it. A 2E26 was used to drive it in one ancient Motorola radio I am familiar with and once owned.
 
Craig,

I don't enjoy raining on somebody else's parade. However, an important point that has only been hinted at has to be made.

How do you propose to convert the "12" VDC in the car's electrical system into the "450" VDC needed by EL34s?

For the pentode/triode in the Dyna circuit, use the 7059. The 7059 is a "12" V. heater version of the 6U8 intended specifically for automotive service. ;)
 

7N7

Member
2003-01-18 10:43 pm
England
Eli Duttman said:
Craig,

I don't enjoy raining on somebody else's parade. However, an important point that has only been hinted at has to be made.

How do you propose to convert the "12" VDC in the car's electrical system into the "450" VDC needed by EL34s?

For the pentode/triode in the Dyna circuit, use the 7059. The 7059 is a "12" V. heater version of the 6U8 intended specifically for automotive service. ;)

Easy: just buy a SS inverter and build a normal valve amplifier.

7N7
 
Eli Duttman said:
How do you propose to convert the "12" VDC in the car's electrical system into the "450" VDC needed by EL34s?

Didn't somebody on this site have a 300B mobile amplifier that used the inverters out of UPS to generate his local AC?

I would probably try an old 12 VDC supply for a tube ham transceiver, because nobody runs a KWM-2, TR-4, etc., mobile these days. I have a half dozen or so of these mobile supplies because you can get them for next to nothing.

I would look for a Drake DC-3 or DC-4 power supply as their rigs used high perveance sweep tubes. The power supplies for the 6146 based rigs would likely have too high a B+ for most common audio tubes.

Win W5JAG
 
7N7 is right. The modern breed of inxpensive, 12 volt to mod sine power inverters are well suited to operate the home type tube amplifiers that many of us enjoy. I have a 1000 watt Xantrex X-Power inverter and it does a pretty good job powering my MingDa Mc34B, P-P, UL 6L6 amp. The amp sounds good but B+ and output power are reduced because of the supply waveform. I suspect one could match this properly in a from scratch amplifier build by selecting a higher voltage B+ winding.

One could spend more and get a true sine wave inverter and this problem would go away.

Many computer UPS systems contain a sine wave inverter. These can be had for peanuts to the watt on the second hand market compared to `actual` sine wave inverters , especially when the inboard battery needs replacing.
 

ray_moth

diyAudio Moderator Emeritus
2004-01-27 8:55 am
Jakarta
British Leyland was killed before birth, by first, the stupid incompetent Labour government that conceived the idea (it should never have existed) and second, by that ****** Stokes who ran it. End of story.
That's how I remember it, too. Run by a bean-counter who understood the cost of everything and the value of nothing. It certainly was the end of the story! And yes, he was a [email protected]*r.

The 6146 got banged about in many mobile radiotelephones and taxi radios on the 2 meter band (where one tube could make 30 watts RMS FM carrier) and seemed to survive it.
What I meant about the 6146 was that its comact design means that it can easily overheat with the thermal stress of continuous-mode duty, such as in audio amps. It works much better in Class B burst-mode duty, typical in radio communications, where it gets the chance to cool down. Early amps using 6146 were prone to tube overheating and failure. The 6550 superseded it in most designs.
 
ray_moth said:


What I meant about the 6146 was that its comact design means that it can easily overheat with the thermal stress of continuous-mode duty, such as in audio amps. It works much better in Class B burst-mode duty, typical in radio communications, where it gets the chance to cool down. Early amps using 6146 were prone to tube overheating and failure. The 6550 superseded it in most designs.

My suggestion was based on mechanical suitability. You cannot make a physically weak tube more suitable by turning down the applied power, whereas I see no reason why one could not choose to operate a PP pair of 6146 or better yet an P-PP quad of these (per channel) at reduced ratings so they don`t run away.
That would get expensive though so perhaps other tubes might be more practically suited. I`d be tempted to try the 829B myself with a chassis/case design employing an air filter and a cooling blower. You can use a single envelope with regulated screen supply as a complete push pull package, or two separate bottles for P-P. Option two would allow UL operation from the output xfmer taps.
 

Craig D

Member
2007-12-25 6:03 pm
Reply

Thanks Guys for all your advise and recommendations. Regarding the 12vdc up to the required 420vac, I am currently running tests in my car with a cheap inverter from 12vdc up to 220vac, then to power a NAD 3020. I realise it's not tubes but I was wanting to see if there was any hum or noise and so far none. If it does not work properly with the tube amp, I can purchase a true sine wave inverter but they are quite pricey. Alternatively get the mains transformer for the tube amp to have a higher voltage on the secondary. I plan to power the tube heaters from the 12vdc. Connecting 2 tubes in series. If this is a bad idea please let me know. My reasons are to draw less current from the inverter.
Regarding the output tubes. I could obtain 6146 tubes from the internet as they are not available locally. Are their pins identical to an EL34? Besides the 6146, are there other tubes more hardier than the EL34. Tubes like the 6L6GC, KT88 and 6550 are easily obtainable.
Thanks.
 

7N7

Member
2003-01-18 10:43 pm
England
Re: Reply

Craig D said:
Thanks Guys for all your advise and recommendations. Regarding the 12vdc up to the required 420vac, I am currently running tests in my car with a cheap inverter from 12vdc up to 220vac, then to power a NAD 3020. I realise it's not tubes but I was wanting to see if there was any hum or noise and so far none. If it does not work properly with the tube amp, I can purchase a true sine wave inverter but they are quite pricey. Alternatively get the mains transformer for the tube amp to have a higher voltage on the secondary. I plan to power the tube heaters from the 12vdc. Connecting 2 tubes in series. If this is a bad idea please let me know. My reasons are to draw less current from the inverter.
Regarding the output tubes. I could obtain 6146 tubes from the internet as they are not available locally. Are their pins identical to an EL34? Besides the 6146, are there other tubes more hardier than the EL34. Tubes like the 6L6GC, KT88 and 6550 are easily obtainable.
Thanks.

The Sovtek 6L6WXT (also 5881WXT) is excellent and cheap (search ebay) and since these are installed as original equipment in guitar amplifiers I am sure that they would be fine in your car.

7N7