Hoffman AB763 in Deluxe Reverb problem &nbs

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dustie1
Junior tube assistant
Posts: 7
(2/1/04 10:48 am)
 Hoffman AB763 in Deluxe Reverb problem
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 I just installed a Hoffman AB763 kit in my Deluxe Reverb RI.
When I flip the power switch on I get a slight hum.When I then switch on the stanby I get a very excessive ,and loud,motorboating.This is with both volume controls off.
Any help is appreciated.
 
GroundhogKen
Forum Moderator
Posts: 2238
(2/1/04 11:01 am)
 Re: Hoffman AB763 in Deluxe Reverb problem
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 If the hum is there with both volumes AND the reverb turned down, then try pulling out the preamp tubes one at a time (V6 thru V1 ) to see which tubes if any make the noise go away.

Make sure the power tube plates are not glowing red ( more than the normal heater glow ).

The next step with be to measure the voltages on the tube pins.

Preamp tubes: pins 1,2,3,6,7,8
Power tubes: pins 1,3,4,5,6,8.

Ken

 
dustie1
Junior tube assistant
Posts: 7
(2/1/04 11:17 am)
 Re: Hoffman AB763 in Deluxe Reverb problem
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 Thanks ,I'll try pulling the tubes.I bought the kit form Mike at Tone Zone and he is very helpful but not in on Sundays and that's about the only time I have to work on it.
 
dustie1
Junior tube assistant
Posts: 8
(2/1/04 12:17 pm)
 Re: Hoffman AB763 in Deluxe Reverb problem
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 Ken,
When I pulled V6 the noise went away.I have retraced everything going to this tube and it is according to the drawing.One question I had that was not clear to me is about the OT green wire.I have it running to the tip on the speaker jack and then a wire running for that speaker jack lug to the 820 ohm resistor.Is this correct?
I have also checked for continuity from the 220k resistors to the points they are connected to under the board and they check good.
 
6G6
Junior tube assistant
Posts: 67
(2/1/04 2:34 pm)
 Re: Hoffman AB763 in Deluxe Reverb problem
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 Try the tube from V6 in another position and see if the hum follows it or stays with the socket by pulling one at a time again.
 
dustie1
Junior tube assistant
Posts: 9
(2/1/04 2:52 pm)
 Re: Hoffman AB763 in Deluxe Reverb problem
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 I tried swapping tubes before I ever posted .It is V6 not the tube.Here are the voltages.My meter could not read some of the power tube voltages.I'm guessing it is due to the ocsillation.

Fill in the DC voltages for the 9 pin pre amp tubes
Pin        Valve 1        Valve 2        Valve 3        Valve 4        Valve 5        Va 6
1        238        254        415        249        337        198
2        0        0        0        0        181        65
3        2.1        1.9        7.3        2.0        264        99
4        AC        AC        AC        AC        AC        AC
5        AC        AC        AC        AC        AC        AC
6        230        380        413        382        422        373
7        0        256        0        0        334        78
8        2.2        0        7.3        1.8        312        99
9        AC        AC        AC        AC        AC        AC

Fill in the DC voltages for the 8 pin power tubes
Pin        Valve        Valve        Valve         Valve
1        0        0                
2        AC        AC        AC        AC
3                425                
4        425        425                
5                                
6                                
7        AC        AC        AC        AC
8                                


 
6G6
Junior tube assistant
Posts: 68
(2/1/04 3:17 pm)
 Re: Hoffman AB763 in Deluxe Reverb problem
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 All the AC readings should be 6.3V, give or take maybe a half volt. They should be steady and readable.
Since this is AC, I'm guessing it is related to the hum.
Double check your heater wiring.
 
GroundhogKen
Forum Moderator
Posts: 2238
(2/1/04 5:08 pm)
 Re: Hoffman AB763 in Deluxe Reverb problem
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 V2 should match V1, but you have 200+ volts on V2 pin 7.

Recheck you connections ( grounds etc ) around the second volume pot. Check the cathode resistor and the polarity of the cap on V2 pin 8.


Ken
 
dustie1
Junior tube assistant
Posts: 10
(2/7/04 10:48 am)
 Deluxe Reverb still
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 I finally got back to this amp this morning.I have checked every components value on the board and have checked all of the wiring to the tubes and pots and can find nothing wrong.I get this motorboating that can be stoped by pulling V2 and yes I do have over 256V on pin 7 of V2.Any idea what I can do next or where this excess voltage is coming from.

Thanks
 
GroundhogKen
Forum Moderator
Posts: 2243
(2/7/04 12:25 pm)
 Re: Deluxe Reverb still
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 What is the voltage of pins 6, 7 and 8 of V2 when the tube is not in the socket?

When the amp is off and discharged, what is the resistance of pins 7 and 8 to ground? Pin 8 should be 1500 ohms, and pin 7 should vary from 0 to 1 meg depending on the volume pot setting. Is the 3rd leg of that volume pot properly grounded?

Ken
 
dustie1
Junior tube assistant
Posts: 11
(2/7/04 1:17 pm)
 Re: Deluxe Reverb still
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 I fixed the voltage on pin 7 of V2.The pot was grounded but the lug was not tied to grounded to the pot.Now the voltage on pin 7 of V2 is 0 however that did not fix the problem.
The voltages on V2 with no tube are:Pin 6 383VDC
Pin 7 O
Pin 8 0
The resistance to ground on pin 8 is 1.5k and on pin 7 with the pot on 0 it is 40ohms and goes up to a high of 280k with the pot on 8 and than drops to 128k at 10 on the dial.
Measuring the pot itself I get the same values so it appears to not be a 1M pot.Could this cause the motorboating?
Your help is very much appreciated.I am a first year electronics student and I don't know where you guys learn all this stuff.
 
dustie1
Junior tube assistant
Posts: 12
(2/7/04 1:44 pm)
 Re: Deluxe Reverb still
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 I pulled the pot out of the circuit and it measures 1M all the way on.Now I'm really confused.
 
GroundhogKen
Forum Moderator
Posts: 2244
(2/7/04 5:32 pm)
 Re: Deluxe Reverb still
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 Sorry, the upper resistance limit on pin 7 will depend on the volume control AND the tone controls.

Put the amp back together. Does the noise change when you spin any of the knobs? Is pulling V2 still the only thing that effects the noise?

Keep checking for wiring mistakes--remember you spotted the ungrounded volume pot AFTER you had checked it over once.

Feel free to re-post the preamp voltages at this point.

Ken
 
PHATamps
I only work on tube amps
Posts: 417
(2/7/04 10:07 pm)
 Re: Deluxe Reverb still
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 OK, you grounded the Reverb channel Volume pot, but V2-b is still not conducting (0V on V2-8).

Double (or triple-quadruple) check your connection between V2-8 and the 1.5K/22uF network, and between that network and ground. Re-solder if necessary (touch up joints anyway).

The voltages on V2-6,7,8 should be about the same as V1-1,2,3; V1-6,7,8; and V2-1,2,3.
 
dustie1
Junior tube assistant
Posts: 13
(2/8/04 8:23 am)
 Deluxe Reverb
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 I'll double check everything again on V2 but I also screwed up on the last post.The only way I can get the motorboating to stop is by pulling V6.
 
GroundhogKen
Forum Moderator
Posts: 2246
(2/8/04 9:39 am)
 Re: Deluxe Reverb
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 Are you sure this isn't the typical positive feedback squeal? Try swapping the two output transformer primary wires that go to pin 3 of the power tubes.

If your OT primary wires are reversed, then the negative feedback line actually carries positive feedback causing a loud squeal like holding a mic in front of a speaker.. Pulling the phase inverter ( V6 ) would make it stop. Anytime you wire up an output tranformer you have a 50% chance of getting it right/wrong.

Ken
 
dustie1
Junior tube assistant
Posts: 14
(2/8/04 10:51 am)
 Re: Deluxe Reverb
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 That is the first thing I tried before posting.This is motorboating for sure.It is so bad that I can't measure the
voltages on pin 3 of the output tubes with my self ranging meter.Yes,the only way of stopping it is to pull V6.
Could it be the filtering?I used the original filter board but removed the last two caps and replaced them with two 22@500VDC caps so I have five 22uf caps.I redid the traces so each positive end has a wire going to a node shown on the drawing and there are two common grounds going to lugs on the chassis.This looks fine to me.
 
GroundhogKen
Forum Moderator
Posts: 2247
(2/8/04 11:11 am)
 Re: Deluxe Reverb
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