bayermar said:was thinking about a 2W resistor that is paralleled with a rotary switch to dial in different parallel resistors. This way there is always at least one resistor (the one with the most feedback) in the path.
Would the sudden change in feedback level that happens when switching "live" be a problem, though?
Do I have to expect a loud "pop" and dead speakers?
I would expect a very slight click or pop, at most. Higher amplitude pops usually come when clipping in capacitance and charging it suddenly. But, if you are worried, test the concept by switching in a parallel resistor, using just a simple on/off switch.
I doubt you'd could blow any but the feeblest of tweeters with a single pop from a modest PP tube amp. But disconnect the tweeter if you want or just use a cheap speaker for the test.
Sheldon
I have a 25K pot in series with a 12K resistor for the shunt feedback on my breadboard mono baby huey so that I can dial in anything from 12K to 37k of shunt feedback. Down around 12K-18K, the vocals/mids seems to be more "up-front" with lots of "air", cranking up the resistance the vocals seem to recede.
I suppose that a parallel arrangement of the fixed R and pot my be more desirable but so far I've been too lazy (or too much of a numb-skull) to figure out the pot and R value for this setup.
A stereo pot and 2 fixed R's would certainly work for both channels...
I suppose that a parallel arrangement of the fixed R and pot my be more desirable but so far I've been too lazy (or too much of a numb-skull) to figure out the pot and R value for this setup.
A stereo pot and 2 fixed R's would certainly work for both channels...
Thanks Yves, Sheldon, and boywonder.
That helps a lot. Actually just got the last post while writing a reply.
Boywonder: What rating does your 25k pot have. I figured the pot would have to dissipate some power and the high voltage on the pot worries me a bit.
I am not sure how to determine the exact current through the resistor/pot since this depends as far as I understand the concept on the AC swing on the driver triode anodes and the AC voltage swing on the pentode anodes. Correct?
Your 25k pot arrangement is basically what I had in mind and your described effect is exactly what I would have expected.
I might try the switch setup or the pot setup with cheap speakers anyway to be on the safe side.
Thanks again.
Martin
That helps a lot. Actually just got the last post while writing a reply.
Boywonder: What rating does your 25k pot have. I figured the pot would have to dissipate some power and the high voltage on the pot worries me a bit.
I am not sure how to determine the exact current through the resistor/pot since this depends as far as I understand the concept on the AC swing on the driver triode anodes and the AC voltage swing on the pentode anodes. Correct?
Your 25k pot arrangement is basically what I had in mind and your described effect is exactly what I would have expected.
I might try the switch setup or the pot setup with cheap speakers anyway to be on the safe side.
Thanks again.
Martin
If you decide to go the switch setup I think you'd be best off using a rotary make-before-break switch. I had a Mesa Tigris that used adjustable global feedback and I was hot-rodding it and I accidentally put in a standard rotary. Whereas before the change there was no popping, afterwards there was.
bayermar said:
As a rough estimation:
As said above, there is only AC accross the pot, all coming from the output stage anodes.
Let's say your amp delivers 10W and the load is 10K p to p.
Thats means there is some 320 volts rms on the OPT primary.
From here there is a resistive divider (R5 + R12 + R14 in the schemo above) .
The total resistance is around 120K, thus the AC current is 2,7 mA.
The power at R12 (your pot) is 160 mW at full output power ... roughly !
Yves.
Thanks Yves,
this makes things much clearer for me and it starts to make sense now.
I have seen 1/2W dual potentiometer around. I might give them a try. Since there will be always a 10k resistor in series (I don't think I need the "no feedback" setting), the power rating should be sufficiently over-rated.
I will report back how I like it.
@exeric:
Yes, I was thinking about a make-before-break switch originally, or the solution where the switch is connected in parallel to a resistor that always stays in the signal path and you switch in different values of parallel resistors. But I think I like the potentiometer solution much more since there will be no sudden changes and I can dial in the feedback just how I like it.
Who knows, maybe I will realize in the end that I always listen with the same setting, then a simple resistor will be the solution again.
Thanks to everyone for their helpful comments!
Martin
this makes things much clearer for me and it starts to make sense now.
I have seen 1/2W dual potentiometer around. I might give them a try. Since there will be always a 10k resistor in series (I don't think I need the "no feedback" setting), the power rating should be sufficiently over-rated.
I will report back how I like it.
@exeric:
Yes, I was thinking about a make-before-break switch originally, or the solution where the switch is connected in parallel to a resistor that always stays in the signal path and you switch in different values of parallel resistors. But I think I like the potentiometer solution much more since there will be no sudden changes and I can dial in the feedback just how I like it.
Who knows, maybe I will realize in the end that I always listen with the same setting, then a simple resistor will be the solution again.
Thanks to everyone for their helpful comments!
Martin
In testing my baby huey bias block CCS's, I smoked the BC547B transistor and the electrolytic bypass caps got quite warm when I applied -15V. I'm thinking that it may have been due to incorrect polarity of the 15V that I applied as I've checked (and re-checked) my circuit to the schematic.
I'm fresh out of BC547B's but have a few BC547A's and I am wondering if they will work. Hfe for the BC547A is 110 @ 2ma, 5V vs 200 @ 2ma, 5V for the BC547B. I have no clue what the significance of Hfe is.........I can order 547B's if the A version won't work, I was just hoping to save some time.
Are my polar electrolytic caps toast also? They got quite warm, but are not bloated, and read short and one direction and open in hte opposite polarity with an ohmmeter which I am assuming is normal. The power was applied for probably 5 seconds or so.
I'm fresh out of BC547B's but have a few BC547A's and I am wondering if they will work. Hfe for the BC547A is 110 @ 2ma, 5V vs 200 @ 2ma, 5V for the BC547B. I have no clue what the significance of Hfe is.........I can order 547B's if the A version won't work, I was just hoping to save some time.
Are my polar electrolytic caps toast also? They got quite warm, but are not bloated, and read short and one direction and open in hte opposite polarity with an ohmmeter which I am assuming is normal. The power was applied for probably 5 seconds or so.
PSU
Hi all
I think I have gathered it all the last few days I have read this thread .. extensively. I am on fire to build the amp but ...
I right now have two questions that havent been brought up yet, I think. And also some comments.
1.) Isn't there allways a concern about bringing Hi voltage to the anodes before the heaters have done their heating? (cathode stripping) No one have thouht that it's an issue in this thread. If you use a tube as a rectifier it's not a problem, I know.
Is this not a problem?
2.) If you read the data on Hammonds transformers they all have the same specs, 30 - 30khz +-3db expet for the 1650E, recomended to try by Ginger and one of the Yves, here at the forum. Hammond says its only good for 70hz at the bottom end. Thats strange. Any comments?
I therefore would like to try the 1608 as my first PP tubeamp OT. Can it be OK with my alltime high other components:
2 matched Telefunken ECC803S NOS (even matched triodes within)
4 matched Philips SQ E84L NOS (250V 37mA) on a AVO MKIII ?
Maybe I should raise my praier to the Nirvana with a amplimo 130khz ring transformer? My budget doesn't allow that though. (The tubes I allready have but not the transformers)
It seems that Germany is the place to buy OPT for me? Can you recommend something else in the ballpark of prizes that Tubetown have on it's Hammonds?
Cheers // Radoman
Hi all
I think I have gathered it all the last few days I have read this thread .. extensively. I am on fire to build the amp but ...
I right now have two questions that havent been brought up yet, I think. And also some comments.
1.) Isn't there allways a concern about bringing Hi voltage to the anodes before the heaters have done their heating? (cathode stripping) No one have thouht that it's an issue in this thread. If you use a tube as a rectifier it's not a problem, I know.
Is this not a problem?
2.) If you read the data on Hammonds transformers they all have the same specs, 30 - 30khz +-3db expet for the 1650E, recomended to try by Ginger and one of the Yves, here at the forum. Hammond says its only good for 70hz at the bottom end. Thats strange. Any comments?
I therefore would like to try the 1608 as my first PP tubeamp OT. Can it be OK with my alltime high other components:
2 matched Telefunken ECC803S NOS (even matched triodes within)
4 matched Philips SQ E84L NOS (250V 37mA) on a AVO MKIII ?
Maybe I should raise my praier to the Nirvana with a amplimo 130khz ring transformer? My budget doesn't allow that though. (The tubes I allready have but not the transformers)
It seems that Germany is the place to buy OPT for me? Can you recommend something else in the ballpark of prizes that Tubetown have on it's Hammonds?
Cheers // Radoman
Radioman:
I'm a newbie, but here is my 2 cents:
Many folks here do not worry about cathode stripping on most "typical" power amps where the B+ voltage is less than 1Kv or so except for a limited number of tubes where it could be a problem (the EL84 is not one of those tubes and the B+ is around 300-350V).
If you are concerned about it, you can always use tube rectification, or a B+ delay circuit with SS rectification. There are lots of threads here about B+ delay circuits, and lots of opinions about cathode stripping if you do a search.
I believe that Ginger and others favor the Hammond 1650 over the 1608 is simply because it is rated for 15W instead of 10W and won't saturate as readily at higher power output. Depending on your B+ voltage and the lower resistor value in the bias block, the amp puts out 10-12 Watts or so. I have no idea why the 1650 is only rated to 70 hz at the bottom end.
I'm a newbie, but here is my 2 cents:
Many folks here do not worry about cathode stripping on most "typical" power amps where the B+ voltage is less than 1Kv or so except for a limited number of tubes where it could be a problem (the EL84 is not one of those tubes and the B+ is around 300-350V).
If you are concerned about it, you can always use tube rectification, or a B+ delay circuit with SS rectification. There are lots of threads here about B+ delay circuits, and lots of opinions about cathode stripping if you do a search.
I believe that Ginger and others favor the Hammond 1650 over the 1608 is simply because it is rated for 15W instead of 10W and won't saturate as readily at higher power output. Depending on your B+ voltage and the lower resistor value in the bias block, the amp puts out 10-12 Watts or so. I have no idea why the 1650 is only rated to 70 hz at the bottom end.
akirasugai said:
Hello Akira
Triple congratulations. My main obstacle to start on tubes is tubes and transformer set as many others in our country.
Where did you get yours. I´m near to Jabaquara, just about 20 minutes from Tecnotrafo, just in case.
Did you import the tubes yourself?
What are your speakers?
OK. Ufa. Enough.
Thanks in advance for comments ahd happy listenings
I have used both the Hammond 1608 and 1650E on Baby Huey's.
With the High Voltage supply at 300V you get 10.5 Watts rising to about 12 Watts with the high voltage at 350 Volts. I therefore thought the 15W rated 1650E would be better than the 10W rated 1608.
I note that the Hammond website says 70Hz low frequency corner for the 1650E BUT I beleive it is a "typo".
I have a spreadsheet that Hammond provided to me which states that the primary inductance of the 1650E is 63.88H which is identical to the specification for the 1608, my view is that this means the low frequency corner frequencies of the 2 trannies will be identical.
Cheers,
Ian
With the High Voltage supply at 300V you get 10.5 Watts rising to about 12 Watts with the high voltage at 350 Volts. I therefore thought the 15W rated 1650E would be better than the 10W rated 1608.
I note that the Hammond website says 70Hz low frequency corner for the 1650E BUT I beleive it is a "typo".
I have a spreadsheet that Hammond provided to me which states that the primary inductance of the 1650E is 63.88H which is identical to the specification for the 1608, my view is that this means the low frequency corner frequencies of the 2 trannies will be identical.
Cheers,
Ian
Radioman62,
I used the Hammond 1650E in the babyhuey and can confirm
that it goes lower on the test bench than 70Hz.
I checked it with the scope, i didn't write down the figures, but i
recall that the low frequency response was ok (read: in line with what you expect from a transformer with this core size).
I'd go with the 1650E as the core size is bigger than the 1608-core,
saturation will be apparent at higher output levels.
Don't worry about cathode stripping. Build the amp with Solid-State-rectification (easier to accomplish), if everything is ok, you can still convert to tube rectification. I found that SS-rectification
provided a fast solid bass.
Kind regards,
Yves D.
I used the Hammond 1650E in the babyhuey and can confirm
that it goes lower on the test bench than 70Hz.
I checked it with the scope, i didn't write down the figures, but i
recall that the low frequency response was ok (read: in line with what you expect from a transformer with this core size).
I'd go with the 1650E as the core size is bigger than the 1608-core,
saturation will be apparent at higher output levels.
Don't worry about cathode stripping. Build the amp with Solid-State-rectification (easier to accomplish), if everything is ok, you can still convert to tube rectification. I found that SS-rectification
provided a fast solid bass.
Kind regards,
Yves D.
Re: PSU
Some say yes' some say no. For one project I did which used a SS power supply, I separated the heater power from the HV DC. The main concern here was to power up the heaters to allow these to warm up before turning on the HV DC. Since this project used direct coupling, the idea was to get the heaters warm so that there wouldn't be any over volting.
As for the cathode stripping issue, it's still a potentisal problem when using hollow state rectifiers. The AC hits the plate(s) before the heaters warm up. Doesn't seem to be much of a problem there, does it?
Radioman62 said:
Some say yes' some say no. For one project I did which used a SS power supply, I separated the heater power from the HV DC. The main concern here was to power up the heaters to allow these to warm up before turning on the HV DC. Since this project used direct coupling, the idea was to get the heaters warm so that there wouldn't be any over volting.
As for the cathode stripping issue, it's still a potentisal problem when using hollow state rectifiers. The AC hits the plate(s) before the heaters warm up. Doesn't seem to be much of a problem there, does it?
gingertube said:
May be yes, may be no !
Primary inductance is dependent of the permeability of the iron, wich is itself dependent of the induction, wich is dependent of the applied voltage divided by the frequency, the turns number and the size/grade of the iron
It would be a good idea to measure them at the same excitation, I use a vy simple jig to do that:
http://www.dissident-audio.com/OPT_da/Mesures/Lf_et_Cp.gif
Sory for the langage, but figures and equations should be understandable anyway
Oh, and use 377 rather than 314 if your mains are 60Hz
Yves.