• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Anyone recognise this amp?

tubewade

Member
2007-06-03 8:54 pm
Can anyone help with identification of this amplifier?

Simply marked "Grommes Custom" with no other identification. Four 6BQ5 valves, four 6EU7s and one 6CA4. Date codes on everything inside are 1966 and 1967. All valves are RCA. Does anyone know what model this is?

[IMGDEAD]http://img262.imageshack.us/img262/5442/pc150038ns3.jpg[/IMGDEAD]
[IMGDEAD]http://img253.imageshack.us/img253/6549/pc150037qk0.jpg[/IMGDEAD]
 
PRNDL, is (IMO) on target with his remark about medium quality.

Notice the lack of mass in the O/P trafos.

6EU7s contain triodes which are electrically identical to those in 12AX7s. A spiral wound, hum bucking, heater and an "odd" pinout slightly reduce AC heating hum levels, when compared to "vanilla" 'X7s. Selection of the 6EU7 suggests that AC heating is being used in the phono section. :(

With its 150 mA. capability, the 6CA4 rectifier is being stretched to the limit to power that signal tube complement.

Still, that's a decent chassis and useful "iron". I think it's quite possible to rework that unit into a nice piece, including a proper DC heater supply for the phono section and a more capable rectifier tube.
 

tubewade

Member
2007-06-03 8:54 pm
Thank you, kind Gents, for your comments and encouragement.

I thought about the possibility of another manufacturer but I have other Grommes/Precision equipment and this shares the common later Grommes design. The transformer numbers are also TO-xxx, a Grommes designation.

I wish the amp were listed at the Grommes "Past Products" site, but it's not.

This amp was probably a relatively inexpensive model in their lineup at the time. Build quality is good, but they didn't use any extra parts, and went to great lengths to avoid additional parts. Only four small signal valves? Two are used to drive the 6BQ5s with a split load inverter. It uses a measure of positive as well as negative feedback, much like the Harman Kardon Ballad A230 or the Dyna SCA35. All four output valves share a single 100 ohm cathode resistor. The other two 6EU7s are the preamp for all inputs and the selector switch alters the negative feedback circuit to provide for the RIAA curve or to reduce gain for the line level inputs.

The output transformers are small. The rectifier is small. But the voltages and currents in this unit suggest to me that it is probably about 6 or 8 watts per channel. That is enough for me as I have lived with 2 to 4 watt single ended amplifiers for a long time. If the phono section proves noisy then I may have to do some changing in the heater supply as my main interest is phono playback. Meanwhile I'm going over everything in this amp to ensure no surprises when powered up for the first time.

Thanks to all who have responded so far. I am hoping to come up with a model number.