| Author | Message | ||
| Matt McDonald |
I recently picked up this 112 RP 100. This is the 6CA7 style with the 1.5K resistor from pins 4 to ground. It has some issues. I noticed that it doesn't have the volume that 100 watts should have. It also seems to cut the treble frequencies. It sounds really muddy with the Deep switch on. My 112 Sixty-Five seems much louder. It seems like I have to crank the gain to about 6 or 7 with the volume on 10 to get what I would expect from 2 or 3 on my 112 sixty-five with the master all the way up. There is no noticeable difference in volume or tone between HI and LO settings. Other than these volume and tone issues there seems to be no other problems like crackles, pops, hum, etc. Here's what I've checked so far: 1) Put new JJ E34L tubes in and biased to 25mV across the 3.9 ohm resistors. 2) Checked B+ at 668 volts with original Mallory caps. Replaced the 100mF caps and 150K resistors under the can. This only brought B+ up to 683Vdc. (I was hoping to at least get the 700Vdc it's supposed to be but did not, strange?). 3) Checked the zener diode to supply 22Vdc to the Control grid's 220ohm resistors. There was exactly 22Vdc. No Problem. 4) Screen voltage checked out at 363 V. 5) Noticed that somebody had put a MC1458CP after the main inputs (IC 1). All of the other original LM1458N op amps were in place so I switched with one from the Phasor circuit. No noticeable change. 6) Here's the strange thing! I do not notice any difference in volume when switching between HI and LO Power. It really sounds like it is in half power all of the time. So, I checked the switch and it works fine. 7) Just checked the power transformer's red and brown wires (to ground ref) that connect to the Standby/HI/LO switch. On HI both wires measure 343V, and on LO both measure 228V. Does this sound normal? If anybody has any hints as to what I might check next I would appreciate it. I have not replaced all the electrolytics on the board yet, so maybe I'll do that next. Maybe I should change the driver transistors to 2N6488? Please,HELP!! Matt McDonald | ||
| Steve Kennedy |
Your output stage may be fine. Your preamp section may be what is lacking input sensitivity or drive level. First check the front-end's power supply voltages. While rectifiers and filter caps/resistors in the power supply are usually the culprits for low B+ voltage, it could be a lossy power transformer or perhaps the primary or secondary are a bit short on windings. While most MM amps seem to have 700-725Vdc B+, I don't think this is universally true (especially 20-30 years down the road). Have you cleaned your front-panel jacks? The insert (100mV & 3V) jacks have contacts that are in series with several sections of the preamp front-end (before the power amp section) so years of dirt, grime & oxidation can cut output levels under the right circumstances. As far as the High Power/Low power issues goes, You may not notice ANY difference in volume UNLESS you are pushing the output tubes right up to distortion in Low power, then hear it clean-up when switched to high power. The Output Power switch allows you to almost double the output stage's headroom but this does not necessarily translate to any great change in achievable output volume. This switch is primarily intended to allow you to operate the amp at a reduced stress level (lower plate voltage) unless the extra headroom of full plate voltage is required. The ouput tubes last longer, as do the other components in the amp that are subjected to potentially lower temperatures while in the low power setting. The front-panel insert jacks may allow you localize your gain problem (if it does exist within the preamp). Plug an audio signal generator into the amp's input (about 50mV p-p at 1kHz)and turn up the amp gain/volume until you can read 100mV p-p at the output of the first insert jack (using an osilloscope). This is the stated level at this point in the circuit. Then, unplug the oscilloscope from the 100mV jack and measure the output at the 3V insert jack. You should be able to see about 3V p-p. Since these points are at different stages in the preamp you might be able to determine that the problem is before, between or after the circuitry that contains the Insert Jacks. I'm sorry I don't have acutal voltage measurements for this but this should provide you some food for thought and perhaps a new starting point. Steve | ||
| Matt McDonald |
Hey Steve, Thanks for the reply. That is something I hadn't and probably wouldn't have thought of. I'll clean the jacks and let you know if things change. I have found this site to be very helpful. Thank you so much. |
Tue, 03/25/2014 - 09:59
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