Hi V-man,
I did connect a CD amplifier direct (sans pre-amp) to the one 5-10 to hear what it sounded like when I originally fired up the first amp, and it was terrible! In spite of the CD player having a level-control for the output, the 5-10 was indeed over-driven and there were mis-matches evident everywhere. I then connected an old Trio 105 valve FM tuner to the same amp, also direct, and the sound was indeed sweet.
I am now running both amps via an Adcom pre-amp, and everything is sweet once more. I think the mod you suggest is worthwhile however, and will carry it out as the amplifiers are indeed very sensitive.
Thanks for the tip!
-Eric
I did connect a CD amplifier direct (sans pre-amp) to the one 5-10 to hear what it sounded like when I originally fired up the first amp, and it was terrible! In spite of the CD player having a level-control for the output, the 5-10 was indeed over-driven and there were mis-matches evident everywhere. I then connected an old Trio 105 valve FM tuner to the same amp, also direct, and the sound was indeed sweet.
I am now running both amps via an Adcom pre-amp, and everything is sweet once more. I think the mod you suggest is worthwhile however, and will carry it out as the amplifiers are indeed very sensitive.
Thanks for the tip!
-Eric
Mods
If you really want to improve/update your amps, check this out:
http://www.lundahl.se/claus_b.html
http://www.lundahl.se/pdfs/claus_byrith/amplifier_30wpp.pdf
http://www.lundahl.se/pdfs/claus_byrith/appendix_cb.pdf
Also contains some very insightfull information. (Do you say it like that ?
) My 1st language is Afrikaans 
If you really want to improve/update your amps, check this out:
http://www.lundahl.se/claus_b.html
http://www.lundahl.se/pdfs/claus_byrith/amplifier_30wpp.pdf
http://www.lundahl.se/pdfs/claus_byrith/appendix_cb.pdf
Also contains some very insightfull information. (Do you say it like that ?
) My 1st language is Afrikaans
Hi JDev,
Interesting reading! I might do a bit of experimenting with the two new amplifiers I am building. I am definitely going to reduce the gain of the amplifiers slightly, as their input sensitivity is a bit touchy for the Adcom pre-amp that I am currently using on them. OTOH, I haven't tried the Mullard stereo pre-amp yet, I want to give it a bit of a run-over before placing it back in service.
Have you carried out any of these modifications?
-Eric
Interesting reading! I might do a bit of experimenting with the two new amplifiers I am building. I am definitely going to reduce the gain of the amplifiers slightly, as their input sensitivity is a bit touchy for the Adcom pre-amp that I am currently using on them. OTOH, I haven't tried the Mullard stereo pre-amp yet, I want to give it a bit of a run-over before placing it back in service.
Have you carried out any of these modifications?
-Eric
When I got my Mullards it was already modified into a piece of not so good amplifier, so I rebuild it and applied about 80% of those mods. I never heard the origenal 5-20, but with these mods it sounds very good. My main trafo is a bit under rated, so I only get about 17Watts out of the amps, but it is more then enough for me. I also used Auricaps for coupling , diodes instead of tube rectifier (spare some more volts) and new JJ caps for PSU. All resistors are also replaced and new Hammond OT's and all new valves as well. All was deffinately worthwhile.
Here is a "before & after" photo of two of my Mullard 5-20s. I have since stripped & rebuilt the one on the right as well, finished it today The layout is not the standard Mullard one, I preferred having the tubes all in front where I can see them nicely a-glowing...
Modifications include wiring the EF86 in triode mode and removing the cathode bypass electrolytic. Noise level measured at the Speaker outputs with Input shorted are in the order of 3-5mV. The components on the tagboard were all replaced, as was the choke. Mains & Output transformers remain as they were, with a new coat of paint. The chassis has been powder coated & covered with a clear lacquer. The filter electrolytics (2 x 16uF; 2 x 8uF, double cans) are original and still seem to be doing a sterling job, even after 35 years.
I haven't posted a picture of the component side yet, the camera's memory stick was full! I will post this in a later post.
The amplifiers are coupled to a Denon AVR2802 receiver for purposes of a preamp & tuner, and they sound great to my humble ears
-Eric
The layout is not the standard Mullard one, I preferred having the tubes all in front where I can see them nicely a-glowing...Modifications include wiring the EF86 in triode mode and removing the cathode bypass electrolytic. Noise level measured at the Speaker outputs with Input shorted are in the order of 3-5mV. The components on the tagboard were all replaced, as was the choke. Mains & Output transformers remain as they were, with a new coat of paint. The chassis has been powder coated & covered with a clear lacquer. The filter electrolytics (2 x 16uF; 2 x 8uF, double cans) are original and still seem to be doing a sterling job, even after 35 years.
I haven't posted a picture of the component side yet, the camera's memory stick was full! I will post this in a later post.
The amplifiers are coupled to a Denon AVR2802 receiver for purposes of a preamp & tuner, and they sound great to my humble ears
An externally hosted image should be here but it was not working when we last tested it.
-Eric
Nice job, glad you like the endresult.Family_Dog said:Here is a "before & after" photo of two of my Mullard 5-20s. I have since stripped & rebuilt the one on the right as well, finished it today
The amplifiers are coupled to a Denon AVR2802 receiver for purposes of a preamp & tuner, and they sound great to my humble ears
-Eric
I also recently got the Denon AVR1804 and DVD2200, some very nice equipment.
Because I am a glutton for punishment, I decided to build another two Mullard 5-20s. Like my previous amplifiers, these have been slightly modified so that the EF86 operates in Triode mode, but I am still using cathode bias for the EL34s (reason being I don't have a separate suitable transformer for the Bias voltages!).
All the components were relatively easy to obtain, bar the 8uF/500V Filter caps. Here I managed to obtain 50+50uF @ 500V. This presents no problem for the GZ34 rectifier, as it can actually handle up to 60uF. I am usinfg a Cap-Choke-Cap configuration. My Mains transformer is wound at 405-0-405VAC, which is slightly below the value that Mullard recommended. Now, on testing the unit, it works perfectly and sounds Oh, so very nice, but my concern is that the HT voltage is some 50V DC higher than it should be.
This might be caused by two things:
* The EL34s I am using are not new, so they might be slightly down, therefore drawing less current than they should. As I only finished the amplifier about 20 minutes ago and it is past midnight here, I have not done a current check on them yet. The cathode voltages are correct at 32V.
* The higher value electrolytics might be the cause of this as they definitely should provide better voltage stabilzation than the 8uF originally used.
I can reduce the voltage input slightly by using the 0-250V AC tapping of the transformer (currently using the 240V AC tapping). I am reluctant to insert series resistors in the plate circuits of the Rectifier, but if I have to, then I guess about 40-5ohms W/W resistors should do the trick.
Tomorrow, time permitting, I will insert new tubes, measure voltages again, and adjust the AC Mains tapping if I have to. Other than that, the amplifier sounds superb, using Hammond O/P transformers and, - blow me down! - I actually wired the NFB correctly the first time!
After I am satisfied with this amplifier, construction of the 4th amplifier will begin. I intend building just one more after that, then I think it's time to stop. The 5 5-20's are going to be used for music as well as for 5.1 Home Theatre operation. I also have a beautiful pair of Tannoy 15" Monitor Gold speakers waiting for them!
Just felt like rambling...
-Eric
All the components were relatively easy to obtain, bar the 8uF/500V Filter caps. Here I managed to obtain 50+50uF @ 500V. This presents no problem for the GZ34 rectifier, as it can actually handle up to 60uF. I am usinfg a Cap-Choke-Cap configuration. My Mains transformer is wound at 405-0-405VAC, which is slightly below the value that Mullard recommended. Now, on testing the unit, it works perfectly and sounds Oh, so very nice, but my concern is that the HT voltage is some 50V DC higher than it should be.
This might be caused by two things:
* The EL34s I am using are not new, so they might be slightly down, therefore drawing less current than they should. As I only finished the amplifier about 20 minutes ago and it is past midnight here, I have not done a current check on them yet. The cathode voltages are correct at 32V.
* The higher value electrolytics might be the cause of this as they definitely should provide better voltage stabilzation than the 8uF originally used.
I can reduce the voltage input slightly by using the 0-250V AC tapping of the transformer (currently using the 240V AC tapping). I am reluctant to insert series resistors in the plate circuits of the Rectifier, but if I have to, then I guess about 40-5ohms W/W resistors should do the trick.
Tomorrow, time permitting, I will insert new tubes, measure voltages again, and adjust the AC Mains tapping if I have to. Other than that, the amplifier sounds superb, using Hammond O/P transformers and, - blow me down! - I actually wired the NFB correctly the first time!
After I am satisfied with this amplifier, construction of the 4th amplifier will begin. I intend building just one more after that, then I think it's time to stop. The 5 5-20's are going to be used for music as well as for 5.1 Home Theatre operation. I also have a beautiful pair of Tannoy 15" Monitor Gold speakers waiting for them!
Just felt like rambling...
-Eric
Oops... told you I was tired http://www.diyaudio.com/forums/images/icons/icon11.gif
Red face
"then I guess about 40-5ohms W/W resistors should do the trick."
should obvioulsy read: "40 - 50 Ohms W/W....."
-Eric
Red face
"then I guess about 40-5ohms W/W resistors should do the trick."
should obvioulsy read: "40 - 50 Ohms W/W....."
-Eric
Today I changed the primary voltage tapping which brought the voltage to exactly where it was supposed to be
However, I had to reduce the 270k series resistor to the EF86 stage to 120k to obtain correct voltages there. This is understandable, because the EF 86 is now wired in triode mode.
This amp rocks! A lot of effort but well worth it
-Eric
However, I had to reduce the 270k series resistor to the EF86 stage to 120k to obtain correct voltages there. This is understandable, because the EF 86 is now wired in triode mode.
This amp rocks! A lot of effort but well worth it
-Eric
Hi,
A friend brought a DIY'd clone of the 5-20 for testing which also works but we found the HT at startup at 490V then falls to 370V after the EL34s have warmed up instead of being in the region of 435V. The voltage across the EL34 470ohms cathode resistor is 26V and 28V respectively. Screens have 1K resistors and are connected to UL taps.
All resistors were tested for correct value, all caps in place (with correct polarity too). Aside from the low HT voltage and low bias the amp works fine.
My first concern is why the HT voltage sags like that? Oh yes, I've replaced the tube rectifier and the 1st and 2nd cap of the CLC filter to 10uf/500V types.
Another observation is that the heaters of the tube rectifier only glows a little, I mean not as bright as the others. Measured the rectifier Vheater at 4.0VAC with tube connected. Would a low heater voltage (for the tube rectifier) cause a large loss of voltage in the HT supply line?
Please help, my brain is getting dried up...
A friend brought a DIY'd clone of the 5-20 for testing which also works but we found the HT at startup at 490V then falls to 370V after the EL34s have warmed up instead of being in the region of 435V. The voltage across the EL34 470ohms cathode resistor is 26V and 28V respectively. Screens have 1K resistors and are connected to UL taps.
All resistors were tested for correct value, all caps in place (with correct polarity too). Aside from the low HT voltage and low bias the amp works fine.
My first concern is why the HT voltage sags like that? Oh yes, I've replaced the tube rectifier and the 1st and 2nd cap of the CLC filter to 10uf/500V types.
Another observation is that the heaters of the tube rectifier only glows a little, I mean not as bright as the others. Measured the rectifier Vheater at 4.0VAC with tube connected. Would a low heater voltage (for the tube rectifier) cause a large loss of voltage in the HT supply line?
Please help, my brain is getting dried up...
SY,
I haven't done that!
My brain trully isn't functioning the way iit should be. 
Anyway, it is a 400-0-400@500mA winding on an EI core. if it does sag, should it be replaced?
Another question on "multi-winding/voltage" transformers, since the 6.3V heaters for the OPT tubes and tube rectifier heaters are on the same tranny, would a sagging winding have an adverse effect on the other windings? I mean for example if the 6.3V heater windings sag under load, would it affect the other windings such as the 5V and HT windings? Or in other words, would it pull down all the other windings causing them to have low voltages under load?
BTW, after playing for an hour or so, the power tranny's EI are hot.
Thanks so much for the help.
JojOD
I haven't done that!
My brain trully isn't functioning the way iit should be. Anyway, it is a 400-0-400@500mA winding on an EI core. if it does sag, should it be replaced?
Another question on "multi-winding/voltage" transformers, since the 6.3V heaters for the OPT tubes and tube rectifier heaters are on the same tranny, would a sagging winding have an adverse effect on the other windings? I mean for example if the 6.3V heater windings sag under load, would it affect the other windings such as the 5V and HT windings? Or in other words, would it pull down all the other windings causing them to have low voltages under load?
BTW, after playing for an hour or so, the power tranny's EI are hot.
Thanks so much for the help.
JojOD
Hi Jojo,
When the amplifier is initaially switched on, the Rectifier is apparently warming up quicker than the other valves in the amplifier, hence the HT rises to 490V off-load but quickly stabilses to a more suitable level as the other valves warm up.
You do not state what rectifier you're using, but if it's a GZ32/34 series, then the filament voltage should be 5V AC. Be careful when measuring this, you're working close to the HT pin on Pin 8.
Are you using any series resistors in the Anode leads between the rectifier & transformer HT windings? If so, what is their value?
Your HT voltage is only some 30-40V low, not a crisis, but it is a bit off spec.
One important fact - what is your AC mains voltage, and what primary tapping of the mains transformer are you using? If you're running at, say, 220VAC on the 250VAC primary tapping, then this will explain your low HT and filament voltages. You did not state what the filament voltages on the EL34s and other valves measured at.
Finally, the Manis transformer could run hot, but not that hot that you could not place your hand on it for 30 seconds or so after the amplifier has been running a while.
HTH,
-Eric
When the amplifier is initaially switched on, the Rectifier is apparently warming up quicker than the other valves in the amplifier, hence the HT rises to 490V off-load but quickly stabilses to a more suitable level as the other valves warm up.
You do not state what rectifier you're using, but if it's a GZ32/34 series, then the filament voltage should be 5V AC. Be careful when measuring this, you're working close to the HT pin on Pin 8.
Are you using any series resistors in the Anode leads between the rectifier & transformer HT windings? If so, what is their value?
Your HT voltage is only some 30-40V low, not a crisis, but it is a bit off spec.
One important fact - what is your AC mains voltage, and what primary tapping of the mains transformer are you using? If you're running at, say, 220VAC on the 250VAC primary tapping, then this will explain your low HT and filament voltages. You did not state what the filament voltages on the EL34s and other valves measured at.
Finally, the Manis transformer could run hot, but not that hot that you could not place your hand on it for 30 seconds or so after the amplifier has been running a while.
HTH,
-Eric
SY said:
I did measure/test the power tranny without any connections and I find the 5Vac@3A, 3.15-0-3.15@5A, and another 3.15-0-3.15@5A to have almost 0 dc resistance! the 400-0-400@500mA is at 46 ohms. The primary winding (220Vac) has an 8 ohms reading.
Now it's blowing fuses up. Even without any tubes in it.
Family_Dog said:
I'm using GZ34 but I tried using 5AR4 and 5U4GB but to no avail.
The anodes of the rectifier is directly connected to the 400-0-400 taps of the power tranny.
The mains voltage here is 220V but realistically is around 215Vac and the tranny is spe'd at 220V.
I remember testing the amp for about 2 hours and the tranny's core is really hot, I may not have the highest tolerance for pain but I can't surely put my fingers for more than 5 seconds. It is really hot.
I'd be grateful for any suggestion, thanks!
Hi Jojo,
From what you're saying, it would seem that the transformer has an internal short, most probably in the Primary winding. The resistance readings you state seem correct, and a shorted turn in the Primary would never show up in a simple resistance test. Have you tried the "Nose Test", sniffing to see if you can smell whether anything has burnt? (With the mains unplugged!).
However, I gather you say the transformer heats up without any load connected to it at all, which would indicate an internal short circuit. Most unusual, but probably caused by a defective valve or cap or even lightning at sometime in the past.
The ratings you provided are more than adequate for the 5-20, in fact, I have the same rating in a single transformer powering two 5-20s. It is not clear why the original transformer should have shorted, you have something like a 100% safety factor there! It would be wise to include limiting resistors between each of the 410V legs of the transformer to the anodes of the GZ34, I would suggest starting with 100 to 150 ohms Wire-wound at 10W. These resistors will get hot so ensure they are placed where they can do no harm and have adequate ventilation.
Transformers can be rewound relatively inexpensively, you do not really need the power ratings you mentioned above if the transformer is only powering one amplifier. A smaller rating should be cheaper to rewind/replace.
The Mullard spec calls for a transformer having the following:
Primary to suit your AC voltage, would suggest -10, 0, 220, 240VAC.
HT: 410 - 0 - 410V @ 200mA
LT1: 3.15 - 0 - 3.15 4A
LT2: 0 - 5V 3A for the rectifier.
If you power a Tuner or pre-amp, then you would have to add another filament winding of anything from 2 to 4 amps, the HT is adequate.
-Eric
From what you're saying, it would seem that the transformer has an internal short, most probably in the Primary winding. The resistance readings you state seem correct, and a shorted turn in the Primary would never show up in a simple resistance test. Have you tried the "Nose Test", sniffing to see if you can smell whether anything has burnt? (With the mains unplugged!).
However, I gather you say the transformer heats up without any load connected to it at all, which would indicate an internal short circuit. Most unusual, but probably caused by a defective valve or cap or even lightning at sometime in the past.
The ratings you provided are more than adequate for the 5-20, in fact, I have the same rating in a single transformer powering two 5-20s. It is not clear why the original transformer should have shorted, you have something like a 100% safety factor there! It would be wise to include limiting resistors between each of the 410V legs of the transformer to the anodes of the GZ34, I would suggest starting with 100 to 150 ohms Wire-wound at 10W. These resistors will get hot so ensure they are placed where they can do no harm and have adequate ventilation.
Transformers can be rewound relatively inexpensively, you do not really need the power ratings you mentioned above if the transformer is only powering one amplifier. A smaller rating should be cheaper to rewind/replace.
The Mullard spec calls for a transformer having the following:
Primary to suit your AC voltage, would suggest -10, 0, 220, 240VAC.
HT: 410 - 0 - 410V @ 200mA
LT1: 3.15 - 0 - 3.15 4A
LT2: 0 - 5V 3A for the rectifier.
If you power a Tuner or pre-amp, then you would have to add another filament winding of anything from 2 to 4 amps, the HT is adequate.
-Eric
problem solved...
changing the power tranny proved to be effective since the B+ now has increased to 420Vdc. it seems that the old tranny sags under load.
anyway, i measured voltages at the chathodes of the EL34 and one has 30V and the other 32V. I removed all the tubes and measured the cathode resistors and they have different values, one has 450 ohms and the other has 475 ohms as supposed to the required 470 ohms.
could this different resistance readings lead to the voltage imbalance on the cathodes of the EL34s? do these different bias voltage readings cause distortion? though I plan to change them (cathode resistors) with tested and close match ones for a more closer bias readings. oh, i did try to interchange the EL34 but the readings remained the same.
thanks!
changing the power tranny proved to be effective since the B+ now has increased to 420Vdc. it seems that the old tranny sags under load.
anyway, i measured voltages at the chathodes of the EL34 and one has 30V and the other 32V. I removed all the tubes and measured the cathode resistors and they have different values, one has 450 ohms and the other has 475 ohms as supposed to the required 470 ohms.
could this different resistance readings lead to the voltage imbalance on the cathodes of the EL34s? do these different bias voltage readings cause distortion? though I plan to change them (cathode resistors) with tested and close match ones for a more closer bias readings. oh, i did try to interchange the EL34 but the readings remained the same.
thanks!
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