Gretsch Pro Bass 6170

CAUTION

LETHAL VOLTAGES PRESENT THROUGHOUT THE CHASSIS

Thanks to my old friend Robert Harper for digging out this amp for us to have a look at. It's the same one being used on stage in 1979 at Apollo Park, Lancaster, CA on the Vox Grenadier XII page. Component date codes put manufacture some time around 1968 and Rob bought it secondhand in Camden, Arkansas in 1972.

1) Introduction

 

The Pro Bass is a 35watt bass combo built for Gretsch by Valco. According to Wikipedia, Valco got into trouble after taking over Kay and was closed down by October 69. There are at least two different versions. The (probably) later model had all the pots grouped together, rocker type  power switches and a push button reset switch instead of a fuse. Labelled as a Pro Bass 6170 Bass/Organ Amplifier, it had square edges, plain covering and black speaker grill that gave a nod to the Seventies and could almost be from a different manufacturer. Have a look at this Youtube clip to see the earlier amp in action with a six string.

 

https://youtu.be/RyOBWc3SGmg

 

The rear ported cabinet has a single 15" speaker with no cover on the bottom of the electronics (it's not a good idea to use it with the back off). Likewise there doesn't seem to have been a protective cover across the valves or anywhere to store the mains cable.

Rob's unit, serial number 2-35188, is currently on it's third speaker after destroying the first two. We don't know what the originals were but the one that survives is an Eminence. The catalogue describes the original as a "Maximum Performance" part (thanks to http://www.vintageguitarandbass.com/ for the info) so that probably means whoever was the cheapest OEM at the time.

After a (not very long) stay in a damp shed it obviously has issues, but it's still salvageable and worth some effort (the spiders and dead beetles inside are straight out of Indiana Jones).

One of the valve bases has been replaced after what looks like a small fire and the mains switch is non original (Rob sat on it). The switch is a three way On/Off/Reverse type designed to work with the US mains of the time. It's in a daft place and bound to get knocked.

 

NOTE:  This one only works at 117v. Check US imports from ebay or wherever before plugging them in.

 

Main job is the rust on the chassis and front panel. The pots are seized solid as well. That plus new electrolytics and a few joint repairs should see it back up and running. Honest !

Make your own mind up what to do with it, but the white material inside the 12AX7 covers is probably asbestos. At the very least leave it alone.

I replaced both of them.

2) Schematic

 

NB: The component numbers, C1 R1 etc, are for convenience only and not factory originals.

 

Power output is 35watts into 8ohms (if this one's got the right speaker in it) from two  6L6s and two 12AX7 (ECC83) preamp valves.

As in some other Valco designs, there's an extra electrolytic (C13) across the main can in the screen grid supply. C15 though (across the 20uF section) looks new and probably shouldn't be there.

 

The front end is a very simple, 3 input mixer and it looks like someone's added the three 1M0 resistors strapped across the jack sockets. 

 

The 5A fuse found fitted in FS1 is too big (the front panel mentions 1.7amps at 117volts 60Hz) and a T2.0 is probably nearer the mark. Dropping the mains from 240v AC isn't a problem but the 60Hz design could be. Short of changing the transformer, it's suck it and see time. The mains filter capacitor (C17) has been disconnected and the ground wire connection doesn't look factory. The original power input was usually  an unpolarised, two wire cable with no ground connection but it currently has a NEMA 5-15P cable fitted.

 

I haven't double checked the circuit yet so please come back and check the issue number now and again.

3) This is the chassis wiring.

 

It's a bit difficult to follow but, hey, it's free. Screened leads are marked up on the outside with what looks like ink or lacquer, shown in the drawing by coloured blocks. There are getting on for twenty different ground connections to the chassis so weird faults could be corrosion related.

 

4) Oh no, not again (again)

 

Just started cleaning up the cabinet and look who's moved in.

 

Plan A is two coats of woodworm killer, two coats of hardener and repair it with wood filler. The good news is that the whole cabinet is solid pine and not plywood.

That didn't work, the whole front is Swiss cheese. On to Plan B ... swear a lot and get the saw out. Same old problem though, the old wood is thicker than the new stuff.

 

Going in to watch the telly now and have a think.

Wife to the rescue. She came home with part of an old teak bench left to rot in the park after the vandals had finished with it (She's good like that).

It took three days to dry out but it's made a nice solid repair.

Left: all back together.

5) Capacitor Stuffing

Although all of the electrolytics need changing after nearly fifty years, it would be nice to keep the layout looking old. Started by replacing C11, C14 and C16, the three caps in one can. 

The originals were 10+10+20uF at 450v, 85degC with unspecified ripple current. The replacements are a compromise between value, ripple and actually fitting inside the can. Lifetime hours quoted are when performance starts to degrade after running at maximum ratings (which they're not). They won't suddenly stop working.

 

LEFT: Back together. 

C11 and C14: Nichicon ULD2W100MPD1TD (Mouser 647-ULD2W100MPD1TD) 10uF, 450v, 20%, 105degC, 92mA ripple, 20000hours.

C16: Nichicon UCS2W180MND9 (Mouser 647-UCS2W180MND9) 18uF, 450v, 20%, 105degC, 220mA ripple, 10000hours.

Before rolling the can edge back over it needs a spacer (the white circle in the central picture above) to replace the original sealant. The ring carrying the twist tabs is a separate part.

6) Together again (as Buck Owens once said)

Reassembled and waiting for test. Still a rat's nest but with new electrolytics, coupling caps and a tidy up.

On the subject of capacitors, the amp has a lot of Z5U discs that people will tell you 'sound awful' and need to be 'upgraded'.

Well each to his own but, unless they're faulty and given that it's supposed to be identical to any number of Supro Holy Grail circuits, why would you want to change the sound? 

7) Switch on

 

Running it up on a variac, there were no apparent problems and the sound through it's own 15" Eminence was outstanding. As it was designed as a bass amp, the tone controls need some thought if you want to use it with a six string. The Bass pot, for instance, rolls the top end off slightly as well as bringing up the bass. Not a problem though.

Closer inspection starts to show up the effects of old age and damp. There have been major value changes in most of the carbon resistors (R17 is +44% and R16 +31%). That makes it impossible to specify what the DC voltages would have been when the unit was new without replacing them all and so no measured values are shown on the schematic. None of this seems to have affected the performance very much and there's plenty of clean signal right up to the 6L6 grids.

 

These are the voltages as found with the old resistors in place:

V1 pin 1 (anode) 228v

V1 pin 3 (cathode) 2.1v

V2 pin 6 (anode 2) 167v

V2 pin 8 (cathode 2) 1.4v

V2 pin 1 (anode 1) 197v

V2 pin 3 (cathode 1) 1.7v

V3 pin 3 (anode) 389v

V3 and V4 pin 8 (cathodes) 28v

V4 pin 3 (anode) 389v

C11 343v

C14 386v

C16 395v

 

The output transformer primary impedance is just over 5k plate to plate.

As far as power is concerned, Gretsch were being a bit optimistic with 35watts. Mind you they didn't say how it was measured. At 117v 50Hz, it makes just under 30watts at about 8 to 10% distortion. Continuous sine wave testing probably isn't the best way to judge a guitar amplifier though and it handles real world signals very well.

8) Back In Business

 

Here's the finished amp together with the Ventura bass that Rob bought with it back in 1972. It actually looks much better in real life than in the photograph, but then don't we all.

The juke box is a 1977 Rock-Ola 469.

 

The amp and guitar are now on their way to Tennessee and a proper 60Hz supply.

© 2015 by backfromthesixties.co.uk