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Tweed Deluxe Sag and Cut-Out

7/4/1999 0:42 AM
Mook	Tweed Deluxe Sag and Cut-Out I have a DIY Tweed Deluxe, Hammond 272 PT The 6V6 plates have about 308V at idle. When I crank the amp to full volume and hit a big chord (like a Pete Townsend Windmill), I can see the plate voltage shoot to about 318, then the plate voltage sinks to about 295. At this point, it seems like a "normal" sag. But then, the amp totally cuts off - no hum, no sound, nothing - for about a tenth of a second and then voltage and hum resume to "normal" idle. First filter is 40uF to a 10watt 5K resistor which feeds the second filter which is 20uF. From the second filter, I have a 10 Watt 20K resistor to the last filter, which is a 20uF. Is parasitic oscillation making it cut-out? Ideas? Thanks a bunch, Mook

7/4/1999 0:42 AM

Mook

Tweed Deluxe Sag and Cut-Out

I have a DIY Tweed Deluxe, Hammond 272 PT

The 6V6 plates have about 308V at idle. When I crank the amp to full volume and hit a big chord (like a Pete Townsend Windmill), I can see the plate voltage shoot to about 318, then the plate voltage sinks to about 295. At this point, it seems like a "normal" sag. But then, the amp totally cuts off - no hum, no sound, nothing - for about a tenth of a second and then voltage and hum resume to "normal" idle. First filter is 40uF to a 10watt 5K resistor which feeds the second filter which is 20uF. From the second filter, I have a 10 Watt 20K resistor to the last filter, which is a 20uF.

Is parasitic oscillation making it cut-out?

Ideas?

Thanks a bunch,

Mook

7/4/1999 4:41 PM
Bruce	Hi Mook, you don't accidently have the OT center tap being fed from the first dropping resistor do you? Well, for that matter... any dropping resistor? You didn't mention if you are running a solid state or hollow state rectifier. If the amp is built like a true tweed Deluxe, the 2 power tubes will be biased with pretty high current at near 12-16 watts each. Close to max. Most folks think it is a class A amp and it is... most of the time. The cathodyne driver section still will be able to hit them hard enough to go into class AB. The 272 (310vac@70ma) Hammond should be OK during this "most of the time" use too, but if yours is idling at 308 (true platev oltage) and you have both 6V6s biased up to the 12-16 watt range, then I think you've maxed out your PT at idle. 24-28 watts at idle and 308vdc is 78ma to 100ma. And that doesn't include the preamp tubes nor I^2R losses. I'd consider resetting the bias resistor to get the current down to fit the transformer, if that's the case. The Hammond 272 is probably a much better choice for a hot rodded Champ using a 5AR4 or maybe SS rectifier... or a class AB1 PP 6V6@30ma ea Princeton type amp. >>>>> But regardless of all that... It also could be the one of .1 coupling caps from the cathodyne driver momentarily charging up with negative voltage and cutting off the power tubes. I use .047uF caps in my version and also 180K grid load resistors on the 6V6s. I don't remember if you said you have an O'scope, but if you do, it's real easy to see these ugly waveforms when they crash from grid sticktion (conduction), over drive signals...etc. BTW...I'm running a little more plate voltage then you are, but I measure them from cathode to anode... abt 330vdc. That means my actual B+ is closer to +360vdc. This is with a 290v@120ma transformer though. The 120ma transformer gives the supply a little more umph and current headroom. How was your 308 voltage derived? Bruce

7/4/1999 4:41 PM

Bruce

Hi Mook, you don't accidently have the OT center tap being fed from the first dropping resistor do you? Well, for that matter... any dropping resistor?

You didn't mention if you are running a solid state or hollow state rectifier.

If the amp is built like a true tweed Deluxe, the 2 power tubes will be biased with pretty high current at near 12-16 watts each. Close to max.

Most folks think it is a class A amp and it is...

most of the time.

The cathodyne driver section still will be able to hit them hard enough to go into class AB.

The 272 (310vac@70ma) Hammond should be OK during this "most of the time" use too, but if yours is idling at 308 (true platev oltage) and you have both 6V6s biased up to the 12-16 watt range, then I think you've maxed out your PT at idle.

24-28 watts at idle and 308vdc is 78ma to 100ma.

And that doesn't include the preamp tubes nor I^2R losses.

I'd consider resetting the bias resistor to get the current down to fit the transformer, if that's the case.

The Hammond 272 is probably a much better choice for a hot rodded Champ using a 5AR4 or maybe SS rectifier... or a class AB1 PP 6V6@30ma ea Princeton type amp.

>>>>>

But regardless of all that...

It also could be the one of .1 coupling caps from the cathodyne driver momentarily charging up with negative voltage and cutting off the power tubes.

I use .047uF caps in my version and also 180K grid load resistors on the 6V6s.

I don't remember if you said you have an O'scope, but if you do, it's real easy to see these ugly waveforms when they crash from grid sticktion (conduction), over drive signals...etc.

BTW...I'm running a little more plate voltage then you are, but I measure them from cathode to anode... abt 330vdc.

That means my actual B+ is closer to +360vdc.

This is with a 290v@120ma transformer though.

The 120ma transformer gives the supply a little more umph and current headroom.

How was your 308 voltage derived?

Bruce

7/5/1999 9:30 PM
Mook	Hi Mook, you don't accidently have the OT center tap being fed from the first dropping resistor do you? Well, for that matter... any dropping resistor? Mook-> Well, I don't know...does this count: The OT center is soldered to the 40uF cap. Also from the 40uF cap is the 5K dropping resistor, which goes to the 20uF cap. So, the dropping resistor and OT center are soldered at the + terminal of the 40uF cap (Mallory Can of 20/20/20/20). I'm using two 20uF sections in parallel. You didn't mention if you are running a solid state or hollow state rectifier. Mook-> It's a 5Y3. If the amp is built like a true tweed Deluxe, the 2 power tubes will be biased with pretty high current at near 12-16 watts each. Close to max. Most folks think it is a class A amp and it is... most of the time. The cathodyne driver section still will be able to hit them hard enough to go into class AB. Mook-> It's not entirely a tweed deluxe. I only used 1 guitar input, 1 volume and 1 tone. Since I did this, I had 1/2 a 12ax7 left over, so I tied the 1st input (12ax7) in parallel. That is, the 1st input, plate is tied to plate, grid to grid, and cathode to cathode; like a giant single triode. I think perhaps some oscillation is happening here. I might have to bridge a 500pf or 1000pF cap from plate to cathode to "bleed" off some higher frequencies. The 272 (310vac@70ma) Hammond should be OK during this "most of the time" use too, but if yours is idling at 308 (true platev oltage) and you have both 6V6s biased up to the 12-16 watt range, then I think you've maxed out your PT at idle. Mook-> It's the 272BX which is rated 300-0-300, 100mA DC. 24-28 watts at idle and 308vdc is 78ma to 100ma. And that doesn't include the preamp tubes nor I^2R losses. I'd consider resetting the bias resistor to get the current down to fit the transformer, if that's the case. The Hammond 272 is probably a much better choice for a hot rodded Champ using a 5AR4 or maybe SS rectifier... or a class AB1 PP 6V6@30ma ea Princeton type amp. >>>>> But regardless of all that... It also could be the one of .1 coupling caps from the cathodyne driver momentarily charging up with negative voltage and cutting off the power tubes. I use .047uF caps in my version and also 180K grid load resistors on the 6V6s. Mook-> I'm using .022 coupling caps, throughout. I don't remember if you said you have an O'scope, but if you do, it's real easy to see these ugly waveforms when they crash from grid sticktion (conduction), over drive signals...etc. Mook-> The only test tool I have is a cheap Radio Shack VOM. BTW...I'm running a little more plate voltage then you are, but I measure them from cathode to anode... abt 330vdc. That means my actual B+ is closer to +360vdc. This is with a 290v@120ma transformer though. The 120ma transformer gives the supply a little more umph and current headroom. How was your 308 voltage derived? Mook-> Simply by putting the + terminal of the VOM to Plate. The - terminal went to ground. I understand this is not "actual" Plate voltage, 'cause it doesn't take into account the opposting cathode voltage. Cathode voltage is about 15V, so my "real" plate voltage would be about 293V. Thanks, Mook

7/5/1999 9:30 PM

Mook

Hi Mook, you don't accidently have the OT center tap being fed from the first dropping resistor do you? Well, for that matter... any dropping resistor?

Mook-> Well, I don't know...does this count: The OT center is soldered to the 40uF cap. Also from the 40uF cap is the 5K dropping resistor, which goes to the 20uF cap. So, the dropping resistor and OT center are soldered at the + terminal of the 40uF cap (Mallory Can of 20/20/20/20). I'm using two 20uF sections in parallel.

You didn't mention if you are running a solid state or hollow state rectifier.

Mook-> It's a 5Y3.

If the amp is built like a true tweed Deluxe, the 2 power tubes will be biased with pretty high current at near 12-16 watts each. Close to max.

Most folks think it is a class A amp and it is...

most of the time.

The cathodyne driver section still will be able to hit them hard enough to go into class AB.

Mook-> It's not entirely a tweed deluxe. I only used 1 guitar input, 1 volume and 1 tone. Since I did this, I had 1/2 a 12ax7 left over, so I tied the 1st input (12ax7) in parallel. That is, the 1st input, plate is tied to plate, grid to grid, and cathode to cathode; like a *giant* single triode. I think perhaps some oscillation is happening here. I might have to bridge a 500pf or 1000pF cap from plate to cathode to "bleed" off some higher frequencies.

The 272 (310vac@70ma) Hammond should be OK during this "most of the time" use too, but if yours is idling at 308 (true platev oltage) and you have both 6V6s biased up to the 12-16 watt range, then I think you've maxed out your PT at idle.

Mook-> It's the 272BX which is rated 300-0-300, 100mA DC.

24-28 watts at idle and 308vdc is 78ma to 100ma.

And that doesn't include the preamp tubes nor I^2R losses.

I'd consider resetting the bias resistor to get the current down to fit the transformer, if that's the case.

The Hammond 272 is probably a much better choice for a hot rodded Champ using a 5AR4 or maybe SS rectifier... or a class AB1 PP 6V6@30ma ea Princeton type amp.

>>>>>

But regardless of all that...

It also could be the one of .1 coupling caps from the cathodyne driver momentarily charging up with negative voltage and cutting off the power tubes.

I use .047uF caps in my version and also 180K grid load resistors on the 6V6s.

Mook-> I'm using .022 coupling caps, throughout.

I don't remember if you said you have an O'scope, but if you do, it's real easy to see these ugly waveforms when they crash from grid sticktion (conduction), over drive signals...etc.

Mook-> The only test tool I have is a cheap Radio Shack VOM.

BTW...I'm running a little more plate voltage then you are, but I measure them from cathode to anode... abt 330vdc.

That means my actual B+ is closer to +360vdc.

This is with a 290v@120ma transformer though.

The 120ma transformer gives the supply a little more umph and current headroom.

How was your 308 voltage derived?

Mook-> Simply by putting the + terminal of the VOM to Plate. The - terminal went to ground. I understand this is not "actual" Plate voltage, 'cause it doesn't take into account the opposting cathode voltage. Cathode voltage is about 15V, so my "real" plate voltage would be about 293V.

Thanks,

Mook

7/6/1999 1:59 AM
Bruce	>> Mook-> Well, I don't know...does this count: The OT >center is soldered to the 40uF cap. Also from the 40uF cap >is the 5K dropping resistor, which goes to the 20uF cap. >So, the dropping resistor and OT center are soldered at >the + terminal of the 40uF cap (Mallory Can of >20/20/20/20). I'm using two 20uF sections in parallel. Nope.. that sounds right.... as long as there is no resistor from the B+ to the OT. >>Mook-> It's a 5Y3. OK Try a 5AR4 here. >>Mook-> It's not entirely a tweed deluxe. I only used 1 >guitar input, 1 volume and 1 tone. Since I did this, I had >1/2 a 12ax7 left over, so I tied the 1st input (12ax7) in >parallel. That is, the 1st input, plate is tied to plate, >grid to grid, and cathode to cathode; like a giant >single triode. I think perhaps some oscillation is >happening here. I might have to bridge a 500pf or 1000pF >cap from plate to cathode to "bleed" off some higher >frequencies. Nope... not yet. You can stick a 47pF to 100pF across the two legs of the final coupling caps. OK that parallel 12AX7 might be a bad move! These amps worked the way they did because the first preamp tube was a lowly 12AY7 not a 12AX7. You've effectivly supercharged a 4 cyl Nova. If you must do this, be prepared for major driver rash and possible overdrive hell. Try and use a 12AY7 or a 12AT7 and see if that helps. You could also drop the cathode bypass cap on one side to about 3K and use a .5uf bypass cap. If the two triodes use their own plate load resistor make one or both about 56K to 68K to kill some of the nearly unuseable additional gain too. Here's a trick you can try I do on my 15 watt Soulkicker. If you do this right, you can use the same input jack and two seperate volume controls and single tone control (as the original) but use a common point grid connection, like you are, but the two outputs could go to their own volume control resistance. Therie outputs are not in parallel. Then you could use the two volume controls like a mixer and with the right Ck and Rk and plate coupling capacitor, you can make it go punchy, bright bassy and thivk with the three controls. >Mook-> It's the 272BX which is rated 300-0-300, 100mA DC. OK! >Mook-> I'm using .022 coupling caps, throughout. That's fine with the extra drive you are using but what is the grid load resistance on the two 6V6s now? >Mook-> The only test tool I have is a cheap Radio Shack >VOM. Been there and done that! >Mook-> Simply by putting the + terminal of the VOM to >Plate. The - terminal went to ground. I understand this is >not "actual" Plate voltage, 'cause it doesn't take into >account the opposting cathode voltage. Cathode voltage is >about 15V, so my "real" plate voltage would be about 293V. Sounds like your Rk on the power tubes is a little small in value. I'd try around 300 ohms and 40uF for quick check. >Thanks, >Mook Hope I can help, Bruce

7/6/1999 1:59 AM

Bruce

>> Mook-> Well, I don't know...does this count: The OT >center is soldered to the 40uF cap. Also from the 40uF cap >is the 5K dropping resistor, which goes to the 20uF cap. >So, the dropping resistor and OT center are soldered at >the + terminal of the 40uF cap (Mallory Can of >20/20/20/20). I'm using two 20uF sections in parallel.

Nope.. that sounds right.... as long as there is no resistor from the B+ to the OT.

>>Mook-> It's a 5Y3.

OK Try a 5AR4 here.

>>Mook-> It's not entirely a tweed deluxe. I only used 1 >guitar input, 1 volume and 1 tone. Since I did this, I had >1/2 a 12ax7 left over, so I tied the 1st input (12ax7) in >parallel. That is, the 1st input, plate is tied to plate, >grid to grid, and cathode to cathode; like a *giant* >single triode. I think perhaps some oscillation is >happening here. I might have to bridge a 500pf or 1000pF >cap from plate to cathode to "bleed" off some higher >frequencies.

Nope... not yet. You can stick a 47pF to 100pF across the two legs of the final coupling caps.

OK that parallel 12AX7 might be a bad move!

These amps worked the way they did because the first preamp tube was a lowly 12AY7 not a 12AX7.

You've effectivly supercharged a 4 cyl Nova.

If you must do this, be prepared for major driver rash and possible overdrive hell.

Try and use a 12AY7 or a 12AT7 and see if that helps.

You could also drop the cathode bypass cap on one side to about 3K and use a .5uf bypass cap. If the two triodes use their own plate load resistor make one or both about 56K to 68K to kill some of the nearly unuseable additional gain too.

Here's a trick you can try I do on my 15 watt Soulkicker.

If you do this right, you can use the same input jack and two seperate volume controls and single tone control (as the original) but use a common point grid connection, like you are, but the two outputs could go to their own volume control resistance. Therie outputs are not in parallel.

Then you could use the two volume controls like a mixer and with the right Ck and Rk and plate coupling capacitor, you can make it go punchy, bright bassy and thivk with the three controls.

>Mook-> It's the 272BX which is rated 300-0-300, 100mA DC.

OK!

>Mook-> I'm using .022 coupling caps, throughout.

That's fine with the extra drive you are using but what is the grid load resistance on the two 6V6s now?

>Mook-> The only test tool I have is a cheap Radio Shack >VOM.

Been there and done that!

>Mook-> Simply by putting the + terminal of the VOM to >Plate. The - terminal went to ground. I understand this is >not "actual" Plate voltage, 'cause it doesn't take into >account the opposting cathode voltage. Cathode voltage is >about 15V, so my "real" plate voltage would be about 293V.

Sounds like your Rk on the power tubes is a little small in value.

I'd try around 300 ohms and 40uF for quick check.

>Thanks,

>Mook

Hope I can help,

Bruce

7/5/1999 6:07 AM
Tracy	Hi Mook I have a 5e3 1959 origional tweed deluxe. If I ever turned that amp on ten I'm sure it would have done the same thing. It has a 5y3 rectifier. Not that thats a bad thing but the amp will sag alot because of the 10K dropping resister between the 1st and second power amp stages. However this lowered voltage on the grid contributes to the tweed sound. Switch the caps to 20uf on the first stage and put the 40uf after the 10k dropping resister. This will hold a tighter grid voltage on the 6v6 . It was a big improvment on my amp.

7/5/1999 6:07 AM

Tracy

Hi Mook

I have a 5e3 1959 origional tweed deluxe. If I ever turned that amp on ten I'm sure it would have done the same thing. It has a 5y3 rectifier. Not that thats a bad thing but the amp will sag alot because of the 10K dropping resister between the 1st and second power amp stages. However this lowered voltage on the grid contributes to the tweed sound. Switch the caps to 20uf on the first stage and put the 40uf after the 10k dropping resister. This will hold a tighter grid voltage on the 6v6 . It was a big improvment

on my amp.

7/5/1999 6:14 AM
Tracy	Mook oops, I mean 5k dropping resister and power supply not power amp.

7/5/1999 9:34 PM
Mook	I'm considering decreasing the dropping resistors. In addition I'll try putting the 40uF on the plates. Can't do this until next weekend, though. Mook

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