Author Topic: Sunn scepter head 1969 trem problem  (Read 4399 times)

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Offline Baddog

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Sunn scepter head 1969 trem problem
« on: April 17, 2011, 05:34:40 pm »
I have the schematic for the amp so I'm set there. I recaped  the amp and replaced the cap can with axial lead caps. I did the totum pole thing for the first filter stage and used 500v 20uf's for the rest . The voltages are a little high in the amp but that to be expected with the raise in wal voltage over the years. Mine has 6550's power tubes with trem and reverb...
The problem I'm having is I can hear the trem pulsing with no guitar signal running through the amp. It pulses with the volume turned all the way down.
Any ideas where to start lookingfor the problem or is their a fix for it?

Thanks
BD

I forgot to say I'm new to this discussion form and sunn amps. I have a 72 fender twin , peavey mace 1/2 stack , crate vc50 3x10 , 72 bassman head and a bunch of mongrel amps I've built from old organ and tone cabinet amplifiers .
« Last Edit: April 18, 2011, 07:36:41 am by EdBass »

Offline Ryan Phelps

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Re: Sunn scepter head 1969 trem problem
« Reply #1 on: April 18, 2011, 10:27:57 am »
Baddog,
Try replacing the three capacitors in the trem circuit. Two are 1 uf and one is 2 uf and they are wired in series. These are often the problem with trem 'ticking" in Fender amps, so it's worth a try.

Good Luck!
Ryan

Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #2 on: April 18, 2011, 10:57:43 am »
I'M having the problem in my sunn amp. Where you referring to the fender or the sunn trem? 

Thanks
Bd

Offline Soundmasterg

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Re: Sunn scepter head 1969 trem problem
« Reply #3 on: April 18, 2011, 10:25:24 pm »
I'M having the problem in my sunn amp. Where you referring to the fender or the sunn trem?  

Thanks
Bd

Almost all of the trem circuits work the same way, regardless of the amp type. There is an oscillator that has the signal come out of the plate and then go through 3 caps with resistors to ground between each, and then the signal gets fed back into the grid of the same tube, causing positive feedback. The cap value determines the speed of the trem, with the .02uf, .01uf, .01uf setup in series being most common. As you go up in value, the speed of the trem slows down. Often when a trem circuit is not working, or if you are hearing ticking noises, then any of those 3 caps or the bypass cap on the stage can be bad. Change those caps, and check voltages on the oscillator tube and see if they are close to spec, keeping in mind the higher wall voltages affecting any voltages in the amp. Some amps will use a cathode follower stage after the oscillator to buffer the output to whatever stage they are affecting. Read the 2nd Gerald Weber book...it has a good section on trem circuits in it.

Greg

Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #4 on: April 19, 2011, 01:56:01 pm »
I believe those are electrolytic caps and I changed them already. Hmmm

Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #5 on: April 19, 2011, 05:19:08 pm »
Ihave another question . What should the voltages be at for the 6an8 tube? What should it be biased at ?
Mine is like the one posted below except no mid boost. I wish the schematic showed the voltages.


http://www.schematicx.com/view-schematic/sunn-sceptre-amplifier-schematic/

Offline Soundmasterg

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Re: Sunn scepter head 1969 trem problem
« Reply #6 on: April 22, 2011, 02:26:51 am »
The Sceptre is using transistors and some different value caps for the oscillator from what is normal on most tube amps. The Sceptre is using a 1uf, a 2uf, and a 1uf for those series caps...are these the ones you changed? They would be on the board in the amp. I don't believe they are electrolytics but its been awhile. I think its the smaller caps on the end of the board, but a pic would help....

The 6AN8 is a triode-pentode, and in this amp it is used as a cathodyne inverter, with the pentode half being the gain stage or driver, and the triode half being the splitter or inverter. I believe the voltages are a little lower than say a 200S, but not by much. The pentode half should have around 90-100v on the plate and the triode should be a bit higher...but again, its been awhile since I saw one of these and didn't write down the correct voltages. I'd look for a plate voltage around there or higher, and a cathode voltage around a volt or two...if the voltages are in those ranges, then I'd suspect a problem somewhere else, especially if you are able to get good signals on a scope out of the phase inverter....

Greg

Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #7 on: April 22, 2011, 05:09:45 am »
Thanks for the reply. I'll check the voltages his morning and post my results. Check out the schematic I posted. If your referring to the three caps that are connected to the pot via resistors . Then they are electrolytic . I did replace them with new ones. Hmmmm


Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #9 on: April 26, 2011, 04:47:02 pm »
HI Greg

6an8 voltage readings
pin 1   300v
pin 6   100v
pin 9     .74v
pin 3   115v

New KT88's biased at 43ma at 537v

The trem gets worse with the NFB loop disconnected..... You can hear the pulse with the volume and trem controls turned all the way down . HMMMMMMM
Did you check out the pics and look at the Electrolytic on the board? They looked stock to me but I could be wrong.  The sceptre schematic I have shows them being electrolytic.... What do you think I should check? Maybe one of the new  I purchased where bad from the factory.... Maybe bad transistor....

Thoughts and suggestions welcome
Thanks
BD

Offline Soundmasterg

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Re: Sunn scepter head 1969 trem problem
« Reply #10 on: April 28, 2011, 03:27:52 am »
HI Greg

6an8 voltage readings
pin 1   300v
pin 6   100v
pin 9     .74v
pin 3   115v

New KT88's biased at 43ma at 537v

The trem gets worse with the NFB loop disconnected..... You can hear the pulse with the volume and trem controls turned all the way down . HMMMMMMM
Did you check out the pics and look at the Electrolytic on the board? They looked stock to me but I could be wrong.  The sceptre schematic I have shows them being electrolytic.... What do you think I should check? Maybe one of the new  I purchased where bad from the factory.... Maybe bad transistor....

Thoughts and suggestions welcome
Thanks
BD

BD,

Those voltages look ok from what I remember on the 6AN8. I don't have anything written down for the Sceptre's that I've worked on, but I do have something written down for my 200S, so I'll check that tomorrow and post if its different by a lot. The voltages are higher and the circuit is different on the 200S, but its close enough to where the voltages shouldn't be too far from that.

You're biased a little on the cold side for a 42 watt KT88...assuming the modern KT88 has the same reading as an old one, but I don't see why that would cause a problem with the trem...just making note of it.

Trem getting worse when NFB is disconnected....seems like it could be just because of the normal gain increase with the NFB loop disconnected.....but pulse with volume and trem turned down.....Maybe the roach is screwed up or the transistors are leaky or something. I'd check and verify all resistors around the trem and around the roach gain stage, and check the caps around that stage too. Something is letting that oscillation go and get amplified when it shouldn't. On the schematic I have, those caps around the oscillator are not shown as electrolytics....so maybe some cheap films or ceramics might work? Ceramics are often used in trems because they don't affect the sound and are more stable under certain circumstances too.  Pretyy confusing but maybe if you take a step back and check some parts....analyze the circuit and how it could be doing what it is doing, then maybe you'll find the problem?

Greg

Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #11 on: April 28, 2011, 08:07:37 am »
Hi Greg
Go back a couple postings and check out the schematic I posted. It shows the caps in the trem curcuit having + symbols on the side of the caps . Hmmm I have a Sceptre schematic that doesn't show them vein polarized .... Not sure what to make of it .... I think I'm going to takethe board out again and desolder one end of every resistor and check the value . Maybe one is open ... I have to se if I have a transistor tester on my multimeter . I looked at Ratshack for transistors and had no luck...
What is a good bias setting for kt88's.
I really appreciate your help with this amp. I want to get it wright so I can enjoy it.
Thanks again Greg
You Rock

Offline Soundmasterg

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Re: Sunn scepter head 1969 trem problem
« Reply #12 on: April 29, 2011, 02:10:21 am »
Hi Greg
Go back a couple postings and check out the schematic I posted. It shows the caps in the trem curcuit having + symbols on the side of the caps . Hmmm I have a Sceptre schematic that doesn't show them vein polarized .... Not sure what to make of it .... I think I'm going to takethe board out again and desolder one end of every resistor and check the value . Maybe one is open ... I have to se if I have a transistor tester on my multimeter . I looked at Ratshack for transistors and had no luck...
What is a good bias setting for kt88's.
I really appreciate your help with this amp. I want to get it wright so I can enjoy it.
Thanks again Greg
You Rock

BD,

I saw a jpg you posted but not a schematic. I can see in the jpg the pic of the amp and the caps in question, and they do look like electrolytics, but there is no reason you have to use an electrolytic as long as the value and voltage rating is correct.

Ratshack is pretty useless these days and has been for like 15 years. We have a local store here that used to be pretty good but even they are getting bad because while they can get you anything you need for the most part, they aren't stocking much of anything anymore which really sucks when you need to visually see the part to see if it will work or not. As time goes on, places like Mouser and AES/CE are going to be where people go to look for stuff all the time instead of the local stores, because no one is stocking anything anymore.

Personally I don't like vintage tube Sunns much for guitar, but I do like them a lot for bass. But if you like it when you get it done, then thats what counts. For the KT88, the vintage spec was 42 watts dissipation, so if you figure 60% to 70% of that, you should have 25.2 to 29.4 watts dissipation. So if the plate voltage stayed at 537v, (which it will not as you adjust the bias hotter) then 50ma would be in the lower to middle end of that range.

Greg

Offline Baddog

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Re: Sunn scepter head 1969 trem problem
« Reply #13 on: April 29, 2011, 06:36:45 am »
If I don't have to work a 12er tomorrow I'm going to camp my but in the basement And fix this dam amp. The truth is this amp is for my brother and he wants it back soon. He uses mostly pedals and just wants and amp witha nice cleans tone . I built him a bassman out of and old late 50's phonograph and it sounds pretty dam good but I think hd wants something that looks a little more like and amp than a robot on stage. I'll post my findings. If it's the trem bug it sounds Ike I'm screwed...

Thanks again Greg