Junker Box amp - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th July 2007, 04:20 AM   #1
diyAudio Member
 
Join Date: Jun 2005
Location: USA
Send a message via AIM to ThSpeakerDude88
Default Junker Box amp

I currently have a 272FX from a project that fell through , as well as a nice 1650F 25 watt OPT. The opt has 40% taps for UL mode as well. I've got a full quad of 7868's , will they run ok on the 7600CT load?

Will the 272fx have enough current to supply the 7868's?

Also, the organ amp that I have ( which was powered by 7868's) had two output transformers on it. They were run with a plate voltage of 450v, grid 420, and cathode resistor 200 ohms. If you look in the datasheet for the 7868 it shows that at a plate voltage of 450v and 170ohm cathode resistor, that the load should be 10k. Is this likley what they are, and is there any way I can be sure?

Also... I have a power x-former from a peavey classic 30 amp. It has a single secondary that apparently after rectification is around 360v, and a 24vac secondary for the bias/heaters ( run in series of cource, one can get creative to work around this voltage).

This is a good x-frmr and I want to use it for something, but I don't know its current rating. The power consumption on the bax of the C30 said 150 watts (!), but the power transformer is pretty small! . I can take pictures of it if you want. The amp was Class AB1 paralell , and used 4 6bq5's for ~30 watts of output. I would not think that the power transformer would be able to run a quad of 7868's, then again the organ said 180 watts max draw, and that is with 30 12au7's and a handfull of preamp and misc. tubes.

The OPT's from the organ amp sound pretty darned good for their size. I would say you could run ( judging by size) a good 15 watts RMS without any severe drop offs in frequency responce. They look a little undersized for 7868's if I am going to be using them for hi-fi, so I was thinking about using 6bq5's instead, as I have about 10 of them already. Will a 10k load work for a class AB1 PP 6bq5 amp? I know the manual shows an 8k load as nominal.

If they are 6600 however, then they would be more suitable for 6v6's.


One last question... Since I have a 272FX and 1650F, Should I just grab another 1650F for $55 , and use a quad of 7868's or 7591's?
__________________
always preach the gospel-
and when necessary use words.
  Reply With Quote
Old 9th July 2007, 12:47 PM   #2
diyAudio Member
 
Tom Bavis's Avatar
 
Join Date: Jun 2002
Location: Macedon NY
272FX is 150 mA - would be OK for stereo 6BQ5s if you use SS rectifier and choke input, since it would be good for about 200 mA. Would be a bit light for 7868s. For an unknown transformer, you can estimate current rating from winding resistance - compare it to the Hammond, for instance.

You can measure the output transformer by applying 120V to the primary and measuring secondary voltage. Impedance ratio is voltage ratio squared. So... if you have 125 in, 5 out, that's 25:1. Impedance ratio is 625:1, 2.5K : 4 Ohms or more likely 5K : 8 Ohms. It helps if you know the speaker impedance, but you can usually guess.
  Reply With Quote
Old 9th July 2007, 05:50 PM   #3
diyAudio Member
 
Join Date: Jun 2005
Location: USA
Send a message via AIM to ThSpeakerDude88
hey thanks for the info. I had not thought of comparing DCR with other known transformers! Good idea. I do know the speaker impedance, it had 8 ohm speakers run off the green tap ( I thought that green was for 4 ohm???) and the yellow taps were taped up.
__________________
always preach the gospel-
and when necessary use words.
  Reply With Quote
Old 9th July 2007, 07:12 PM   #4
diyAudio Member
 
Join Date: Jun 2005
Location: USA
Send a message via AIM to ThSpeakerDude88
I didn't exactly want to put 120 on the primaries just yet, but I did manage to use a smal 10va 13v x-frmr on the secondary. This gave me 456v on the primary.

This turns out to be a ratio of 35.7:1, squared that would be 1229.90, then if I multiply that by 8 ohms that gives me 9839 ohms. If I did his right, this means I have a 10k plate to plate resisance?(which would match the impedance suguested in the handbook for the given voltages and bias )
__________________
always preach the gospel-
and when necessary use words.
  Reply With Quote
Old 9th July 2007, 07:26 PM   #5
diyAudio Member
 
Join Date: Jun 2005
Location: USA
Send a message via AIM to ThSpeakerDude88
I applied wall voltage to the primary and remeasured.

Here are my results:

Wall voltage = 123.9vac
Secondary voltage = 3.9vac

123.9/3.9 = 31.76923077

31.76923077 * 31.76923077 = 1009.284024
1009.284024 * 8 ohms = 8074.2719

This equates to an 8k plate to plate load, which is ideal for 6BQ5's, but not what is reccomended by the manual for cathode biased 7868's @ 450v plate V.

Which of these measures should I believe?



__________________
always preach the gospel-
and when necessary use words.
  Reply With Quote
Old 9th July 2007, 09:14 PM   #6
diyAudio Member
 
Tom Bavis's Avatar
 
Join Date: Jun 2002
Location: Macedon NY
I'd believe what you just measured - the first one might be better, IF you measured the input voltage to the last 0.1V - that's about 1%. The second one has an uncertainty of 0.1V, or 2.5%, which becomes 5% when squared - about 400 Ohms plus or minus...

One more refinement: the ratio was measured without a load. The load seen by the tubes also includes the winding resistance. The primary is easy to measure - for the secondary, assume it reflects a similar value (since transformers are usually designed for a similar copper loss in primary and secondary). So, say the primary measures 200 Ohms DCR - add 400 to your previous value.

I think these will be excellent for 6BQ5s or 7189s, depending on your supply voltage.
  Reply With Quote
Old 10th July 2007, 03:57 AM   #7
diyAudio Member
 
Join Date: Jun 2005
Location: USA
Send a message via AIM to ThSpeakerDude88
My P2P DCR is 315 ohms
__________________
always preach the gospel-
and when necessary use words.
  Reply With Quote
Old 12th July 2007, 03:26 AM   #8
diyAudio Member
 
Join Date: Jun 2005
Location: USA
Send a message via AIM to ThSpeakerDude88
I wanted to ask about my OTP's on here instead of starting a new thread.


Does anyone have experience with the hammond 1650F? If it is better than what I got, I will order another one and build myself a nice stereo amp.
__________________
always preach the gospel-
and when necessary use words.
  Reply With Quote
Old 13th July 2007, 07:19 AM   #9
diyAudio Member
 
Join Date: Oct 2004
Location: Adelaide South Oz
Dude,
The output transformer will be fine and I reckon the power tranny will be as well if you back off the tube current a bit.

7868 - at 450V B+
recommendation is for common 170R cathode resistor and that gives 86 mA per push pull pair or 43 mA each tube. ALSO thats right on the limit for the tubes 19 watt anode dissipation rating.

Instead:
Cathode bias using separate 390R bias resistors on each cathode or a common 180R or even 220R to get the tube currents down to 75mA per pair or 37 mA each - I'd recommend the separte 390R with 470uF or even 1000uF 50V or 63V rated bypass caps.

That will allow the power tranny to cope with 2 channels and will also give better tube life as dissipation is now down around 16.5 watts. With the 8K Raa output trannies that should also give you pretty close to the 25 Watts Output they are rated at.

Go for it.

Cheers,
Ian
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



New To Site? Need Help?

All times are GMT. The time now is 03:17 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2