SF VIbro Champ Really Low Volume, Clean All the Way Up

[kinggabbo]

I just acquired what I think is a '73 Vibro Champ. It sounds great but has really low volume even cranked all the way. It is clean all the way up but is not loud at all. I know it just a 5-6 watt amp but it should get way louder than this is. I am not an expert on amp troubleshooting or reading amp schematics but I think I've found two orange caps that are not stock, along with filter caps inside the chassis, instead of the cap can. One is attached to the filter caps and the other is attached to the power tube from pin 8 to ground I believe. I swapped all tubes with known good tubes, same result

Here are some pics. The caps are 104K 630V. I have removed them. No change.
Here are some voltage measurements I took yesterday. Maybe they'll help.

V1
pin1 262v
pin3 2.2v
pin6 260v
pin8 2.2v

V3 (6V6 Power Tube)
pin3 428v
pin4 432v
pin8 27v

Thanks for any help.




Volume Pot Wiring

[Chuck H]

It looks to me like the preamp tube isn't conducting. Have you tried a different tube? It may help to clean and retension the sockets. Most likely it's a ground fault in the preamp.

Also, an occasional problem in those amps, especially ones that have been worked over, is the black circuit board can become conductive causing a false bias condition. Take a DC read at pin 2 and 7 of V1. If you find DC there then you may have this problem.

[Jazz P Bass]

Is the speaker stock?
From the factory Fender used 4 ohm speakers.
Maybe an 8 ohm was installed.

[Chuck H]

I did a 10" speaker "upgrade" for a guy with a VC a long time ago. Very hard to find 4 ohm 10" guitar speakers at that time (still is). So we installed an 8 ohm speaker. The amp was still pretty loud. This isn't to say the OP may not have a speaker problem as well.

[Chuck H]

Well I just looked at the schem and it would seem that with the higher mains voltage today the V1 voltages are correct. So I would say that a signal trace would be the way to go for starters.

[kinggabbo]

Is the speaker stock?
From the factory Fender used 4 ohm speakers.
Maybe an 8 ohm was installed.

I think the speaker may be stock or at least from the 70's. I think it is a Pyle speaker. 3.2 Ohms I think written on the back and it measures that much as well.

It looks to me like the preamp tube isn't conducting. Have you tried a different tube? It may help to clean and retension the sockets. Most likely it's a ground fault in the preamp.

Also, an occasional problem in those amps, especially ones that have been worked over, is the black circuit board can become conductive causing a false bias condition. Take a DC read at pin 2 and 7 of V1. If you find DC there then you may have this problem.

OK, maybe that's it. Yes I have tried known good tubes, but the first preamp tube socket seems a little angled and not straight up and down like the others when I insert the preamp tube. I'll check that tonight.

[MWJB]

If one of your cathode bypass caps is open (the white caps on the board at pins 3 or 8 of a 12AX7, Pin 8 of the 6V6) you will get a drop in gain.

I'd replace these, along with the filter caps, use discrete 500vdc types for the main (40uf) & screen nodes (20-40uf), grounded to a PT bolt. Use 20uf for the preamp node & ground to the #1 input jack ground lug. Replace the 470ohm 2W resistor (which is dissipating 1.6W) with 470ohm 4-5W.

[hack]

Replace the 470ohm 2W resistor (which is dissipating 1.6W) with 470ohm 4-5W.

Would a ten watt resistor work in this instance?

[52 Bill]

Yes, a ten watt resistor could be used, but it would be harder to fit on the board.

Man, that is an ugly recap job. I hope that was done by the former owner and not by a paid service tech.

Another thing to check is the input jack number 2's normally closed contact. If it is dirty it will also reduce the volume.

[kinggabbo]

Well good news!!! I replaced all of the cathode bypass caps. Now, the amp is considerably louder and I get the most awesome sounding distortion when I crank it to ten. I could hardly quit playing it last night.

I still think it is not quite as loud as it should be, but maybe that is all in my head. Perhaps the speaker has something to do with that as well.

[Gtr_tech]

Plug it into a 4 ohm 4-12 and see how loud it is.....

Percieved loudness is not all in the amp itself, the speaker makes the sound and moves the air. Speaker cone area and effeciency both have alot to do with how loud it is to your ears......

[kinggabbo]

Yeah, I went home and played it again during lunch. I think the volume is just fine on it. The small speaker size has alot to with me thinking it might need to be louder. Probably Pyle speakers are not the best out there as well.

I am loving the tone and happy with the volume. Thanks for everyone's help. I will try to post a sound clip tonight.