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Harmony 306, low volume

Danohss

Member
Messages
32
working on a Harmony 306, whoever worked on this before me should have their soldering iron and all tools taken away from them...
I have spent painstaking hours with the schematic putting this amp back in order, got it almost squared away, sound good but volume is low - half or less than what I'd expect. I think I have the culprit identified but thought I'd seek some advice. V2 (12ax7) - all new tubes, I have 72 VDC on pin 8, the second cathode. Other cathode is 1 VDC. This (the 72)seems high to me, but maybe I'm missing something. I can pull either 6V6 with no real difference so I assume both sides of the power section are OK, its something in the gain stage somewhere. If this amp uses that 2nd half of V2 for a phase inverter, is 72 VDC too high?
 

tele_player

Member
Messages
732
The schematic I found is horrible, very hard to follow the wiring on the 12ax7, but it looks like a split load phase inverter, so cathode voltage is expected to be way higher than on a normal triode gain stage. 72v seems plausible, given the 39k plate and cathode resistors.
 

Garthhog

Member
Messages
135
Your going to need a schematic. Without it, we are flying blind. If the 2nd half of V2 is a cathode follower, you will have substantial voltage at it's cathode. What is the voltage on the corresponding grid?
 

Danohss

Member
Messages
32
I have 2 good schematics - the one on the back of the amp panel is very clear and I found the H306 (not 306 A) on the EL84 page library of schematics which gives me an easy copy to sit at a desk with. I've traced every path comparing to the schematic, everything checks out, I've been over it a bunch of times with a fine tooth comb. I've replaced all the electrolytic and tone caps, many of the resistors which had drifted, new tubes. Some of the voltages I have are:
V1 (12AX7) Plates are 165 and 158 VDC
Cathode - 1 and 1VDC

V2 - Plates 183 and 217
Cathodes 1 and 72

Power tubes - Plate voltage 346
Cathodes - 18 VDC (positive voltage)
Pin 4 - 284 Volts - seems high for a screen/grid?
Pin 5 - 0

When I started it had low volume, a nasty hiss (bad resistors) and the trem circuit was wired all screwed up. Hiss is gone, sound is nice, just low volume. I have many speakers on the shelf so I have tried multiple known good 8 ohm speakers, that's not the problem. All grounds check out with a ohm meter. Installed a grounded 3 prong cord. One other observation is that V1 does not seem to glow as bright as V2, heater voltages check out fine, swapped V1 and V2, low glow stays on V1 socket.
I do not have an oscilloscope unfortunately, its next on the list of bench gear to acquire.
Any thoughts would be greatly appreciated!
 

TimmyP

Member
Messages
2,488
Does it get dirty early, or never get dirty? If the latter, perhaps adding cathode bypass caps on the gain stages would do the trick?
 

Danohss

Member
Messages
32
It never breaks up. I'm also wondering if there's a typo on the schematic - shows 2 legs of the tone pots going to ground (wiper and one leg) - seems counterintuitive to me but I guess it could be right - certain frequencies passed from junction with volume pot thru cap to tone pot, shunted to ground, but looking at the schematic there seems tobe no way for the signal from the tone pot to get back into the signal path. Since the cap to the tone pot is below the volume pot leg, there seems to be no real function to the tone pots as shown - all the signal would also go straight into the volume pot, out through the wiper to the 2nd gain stage. I also checked the O/P tranny, 236 ohms on one side, 198 on the other - Center tap to each side. Still struggling with it, been through the schematic and curcuit over and over, cant find anything wrong. Clearly I'm missing something though...
 

Danohss

Member
Messages
32
By the way, the amp is the 306, not the 306A, schematic I'm working from on the back panel matches the one on the EL34 web site library of schematics.
 

pdf64

Member
Messages
8,134
I can pull either 6V6 with no real difference
When power tubes share a cathode resistor, both tubes must be functional and in place; if either is dead or pulled, the other will be horribly underbiased / overdissipating:eek:

schematic I'm working from on the back panel matches the one on the EL34 web site library of schematics
A link to that would help ensure that we're all working off the same info:aok
 

jthomas666

Member
Messages
681
I realize that this is unlikely, but sometimes a wrong resistor value in the signal path can cause an otherwise functioning amp to lack volume. That also could be a pot problem.
 

TimmyP

Member
Messages
2,488
The DC on the heaters is from one side of the heater supply being connected to the cathodes of the 6V6s. This is to reduce hum from the heaters.

If the cathode resistor is still 250 Ohms, that 17V gives you 68mA shared between the two 6V6s. This may or may not be OK, depending upon the B+ Voltage.
 

zenas

Member
Messages
8,872
Just a thought, something I've noticed on "off brand" vintage amps. They tend to not be as loud as an old Fender with the same tubes. Speakers is one thing and nobody drove tubes like Leo did, blackface Deluxe Reverb for example.
Anyways just a thought.
 

Danohss

Member
Messages
32
I may have found ther culprit - decided to put my bias probe in and see what each power tube was reading - all of the voltage testing I did was from the bottom of the chassis. I did retension all the tube socketrs awhile ago so assumed they were all good, none of the sockets showed any sign of arcing when examined with a high power light and magnifier. Voltages were present on all asocket connections from the underside of thge chassis. Confirmed grounds, value of the bias resistor, etc. When I put the bias probe in, the first tube (V3) showed 18ma which is way off. When I put the probe on V4 I found no current reading. I expect there is a cracked or broken socket somewhere on the V4, I've ordered new sockets which I will install when they arrive. Stay tuned...
 

Danohss

Member
Messages
32
Nope, that wasnt it. What about the screen voltages on the power tubes? I have 353 and 369 vdc on the plates of the power tubes, the screens are at 234. Is that too low? Values of the resistors on the filter cap (can cap) check out, haved replaced them with new and verified the actual values with a VOMeter before installing them. Still baffled...
 

pdf64

Member
Messages
8,134
I have 353 and 369 vdc on the plates of the power tubes, the screens are at 234. Is that too low?
Compare your voltages to those in a 5E3, bearing in mind your amp has an even bigger 10k HT dropper to the screen grid node, so we can expect a bigger voltage drop across it.
Are the 6V6 cathodes still idling at 17V?
 

Jeff Gehring

Gold Supporting Member
Messages
6,797
Nothing to do with pdf64's excellent post above but, as a total side note to do with the 5E3 drawing, it bugs me to see data perpetuated that is misleading. In the 5E3 layout above, Telenut62 shows us that the phase inverter (a cathodyne) will have nominal voltages of 45V cathode, 17V grid, and 200V on the plate. Fine, I guess, from one point of view -- that whoever is taking the voltage measurements is blindly accepting the data without understanding how tubes work.

The issue is that 17V recorded for the grid. Yes, if you slap your meter on there, you may measure 17V or something like it. BUT the tube is not operating normally for as long as the meter is touching the grid. Since this is a very high resistance point in the circuit, the very act of touching that grid with your meter LOADS the circuit down, causing the erroneous reading of 17V.

Think about it: every other triode in the amp is operating with a one point something volt difference between the cathode and grid. If you take the above layout readings at face value, the grid to cathode voltage for the PI is (17V minus 45V) or -28 volts! That triode is very nearly shut off for the period of time when your meter is sitting on its grid.

If you want to REALLY see what the normal operating DC voltage at the cathodyne phase inverter grid is, you can indirectly measure the grid voltage by measuring the voltage to ground from the junction of the 1.5k cathode bias resistor and the 56k cathode load resistor going to ground (since there is practically no DC current flow in the 1M grid resistor that goes to the grid.)

TLDR rant over, back to the Harmony....
 






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