Silvertone 1474 Resuscitation

[Justin Thomas]

G1,

Am I reading this right:
The 39R resistor and the 220R resistor are in series, making a cathode resistor for the 4x6L6s of ~1K, which is not entirely out of line... Just because the 12AX7 heaters are connected to the junction between the two doesn't negate the fact that there is indeed a cathode resistor, and a 10uF bypass cap, too, right?

I didn't spend much time looking at the entire heater circuit, so I won't comment on that. Yet.

Justin

[nickb]

Yes Tom and erobillard, I have this book as well and have read this statement. With the heaters in parallel with R53, 220 ohm, 5W bias resistor and both of them in series with R52, 39 ohm resistor, but having all this bypassed by C23 10uF, how would current through the heaters change? I suspect the cathode bypass cap would keep the current relatively stable since it's basically a short to AC, so current through the heaters remains relatively constant with changing signal, just like bypasing a preamp cathode keeps the bias voltage constant there. Am I in the ballpark with the answer? An unbypassed cathode resistance would be a different story I would think. But I have the stock tubes in the amp from 1962, so there can't be much detriment to the heaters. And the amp still sounds great. It really nails that trashy, early 60's blues distortion when cranked.

The average current through the cathodes will rise as the amp is driven. Since that rise is always in the same direction for all the tubes the bypass cap will not make any difference. When driven hard and operating in class AB each output tube can only add to the current. The cap will slow the rise to the new value but eventually the cap will charge up and average current will increase.

On the 12AX7 as part of the cathode resistance, it's not going to change the tone but it may shorten the heater life somewhat. I suppose you could put a 13V 5W zener across those heaters to prevent that.

[g1]

G1,

Am I reading this right:
The 39R resistor and the 220R resistor are in series, making a cathode resistor for the 4x6L6s of ~1K, which is not entirely out of line... Just because the 12AX7 heaters are connected to the junction between the two doesn't negate the fact that there is indeed a cathode resistor, and a 10uF bypass cap, too, right?

Not sure what you meant about the 1K thing?
Yes there is a cathode resistor and bypass cap. The cathode resistor is the sum of the 39 ohm and the 220R/12AX7 heaters parallel combination.

[Justin Thomas]

Sorry about the confusion. What I meant was, a 260R for all four tubes is roughly equivalent to having a 1K for each, if the resistors were individual? I meant that that value of cathode resistor for a single 6L6 might be "odd," since most seem to use a 500R... But not so far as being incredibly off.

Justin

[g1]

I see what you mean. The heaters in parallel with the 220R will make it quite a bit lower.

[DRH1958]

As long as we've gone down this rabbit hole, what is the resistance of two 12AX7 heaters in series while lit up? I'm trying to get a handle on the final bias resistance value. I suppose you could do the math to get the heater resistance. 12.6 volts for each tube(schematic shows only two connections, so pins 4,5) so 25.2 volts across both tubes in series, divided by .15 amps(current is the same in all points of a series circuit). 25.2/ .15A = 168 ohms for the heaters according to the data sheet numbers. So 168R parallel with 220R = 95.25 ohms for this parallel resistance. 95.25 + 39 = 134.25 ohms total. Does this make sense? Of course, the voltage will change with increased cathode current but there has to be somewhere to start, so I just used the data sheet values.

[g1]

Agree with your math. Measuring the cold heater resistance with a meter would not give an accurate indication of the hot resistance, so you would have to measure voltage and current when hot to calculate an accurate resistance measurement.
The data sheet values should be sufficient, but with modern tubes and legacy data sheets ...

[erobillard]

Fresh tubes and most caps showed up, still awaiting the solid-state rectifier. In the meantime I started on a Silvertone 1474 Parts List , you can find it at: https://1drv.ms/f/s!AsTlnbsw-ik6lv5oMUoxLvB6dxYVrw.

DRH1958: there are a few values on the schematic I couldn't quite read; they're highlighted if you get a moment one day to check for accuracy, no rush. Also thanks for the notes on the 1484; once the 1474 is live I'll give those mods a try.

Cheers,
-Eli.

[DRH1958]

Fresh tubes and most caps showed up, still awaiting the solid-state rectifier. In the meantime I started on a Silvertone 1474 Parts List , you can find it at: https://1drv.ms/f/s!AsTlnbsw-ik6lv5oMUoxLvB6dxYVrw.

DRH1958: there are a few values on the schematic I couldn't quite read; they're highlighted if you get a moment one day to check for accuracy, no rush. Also thanks for the notes on the 1484; once the 1474 is live I'll give those mods a try.

Cheers,
-Eli.

Just so everyone knows, that schematic magnifies twice after clicking on the thunbnail in the post. Excellent idea to get the parts list going, Eli. It will make finding the components easier as well since it identifies their function. It will go a long ways to complimenting the schematic to have a complete info resource. Yes, those highlighted values are consistent with my original schematic. And I didn't record the part number for two components that I drew in by hand on the bottom left of the schemo (Ch 1 preamp). So thanks to your parts list, the .02uF coupling cap off 12AX7 pin 1 is C3 and 3 components to its right, the .0015uF bass cap is C9.

EDIT: Actually three components not identified. The bottom grid stopper, R4 is 68k as well. I thought that to be obvious i guess.

[erobillard]

Update: Think I'm done updating the power stage, just need to check my work against earlier shots. Replaced the electrolytics with Sprague Atoms, and the green 600V Sangamos with 630V Suntans (will they withstand? they were convenient). Someone had replaced a 1.5k 1W resistor with a 3.3K wire wound, and I happened to have exactly one 1.5k 1W carbon comp to bring it back to spec. I may still bring that up to a 5W wire wound (thoughts welcome) - which I thought I'd bought earlier today but let's just say I'm glad I test values before reaching for the solder. I don't consider my work pretty but I will take a photo of the section.

Hoping the SS rectifier shows up tomorrow. I'd really like to leave the other stages as-is for the sake of trying to bring those caps back with a Variac. And also I'd need to order one more 16uF plus a pair of 10uF Spragues if I'm just going to clip 'em all. Must resist temptation to plug in and step through this proper.

I also updated the part list, thanks DBH1958 for confirming values. It's now in 3 worksheets - caps, resistors and counts.

Cheers,
-e.

[DRH1958]

Update: Think I'm done updating the power stage, just need to check my work against earlier shots. Replaced the electrolytics with Sprague Atoms, and the green 600V Sangamos with 630V Suntans (will they withstand? they were convenient). Someone had replaced a 1.5k 1W resistor with a 3.3K wire wound, and I happened to have exactly one 1.5k 1W carbon comp to bring it back to spec. I may still bring that up to a 5W wire wound (thoughts welcome) - which I thought I'd bought earlier today but let's just say I'm glad I test values before reaching for the solder. I don't consider my work pretty but I will take a photo of the section.

Hoping the SS rectifier shows up tomorrow. I'd really like to leave the other stages as-is for the sake of trying to bring those caps back with a Variac. And also I'd need to order one more 16uF plus a pair of 10uF Spragues if I'm just going to clip 'em all. Must resist temptation to plug in and step through this proper.

I also updated the part list, thanks DBH1958 for confirming values. It's now in 3 worksheets - caps, resistors and counts.

Cheers,
-e.

Was that 1.5k resistor that was replaced with the 3.3k wirewound R71? That one seems its the only one of that value.
Nice choice on the Sprague Atoms and those Suntans should be fine. Did you replace all the Sangamos? if so, expect the amp to sound different since vintage caps give a different sound than new ones wnen replacing signal caps. But if they are leaky or way out of spec, you don't have much of a choice (except to buy vintage ones to replace them with like Sprague Black Beauties).

[erobillard]

Yes, here's a more complete (though darker) "before" shot of that section, the 3.3k is the dark, barrel-shaped resistor near top-middle.


Here's a shot of that section now:


The lighter brown 1.5k is there now, the ground switch is gone, the fuse holder, caps, 3-prong and ground post are all new. Still to do: that connecting cap is temporary and haven't tied anything down yet. Resistors all test within reason.

[DRH1958]

Yes, here's a more complete (though darker) "before" shot of that section, the 3.3k is the dark, barrel-shaped resistor near top-middle.
[ATTACH=CONFIG]40605[/ATTACH]

Here's a shot of that section now:
[ATTACH=CONFIG]40604[/ATTACH]

The lighter brown 1.5k is there now, the ground switch is gone, the fuse holder, caps, 3-prong and ground post are all new. Still to do: that connecting cap is temporary and haven't tied anything down yet. Resistors all test within reason.

You shouldn't need to replace it with a higher wattage. Here's the math: 20 volts dropped across the 1.5k, 360 in, 340 out =20. 20/1500= 13.3mA. 20V X .0133A= .267 watts. It should increase since these are DC idle voltages. But since it comes to just over 1/4 watt, you have plenty of wiggle room, so 1 Watt should do it. You could take a measurement when it's under load if you feel comfortable doing that with it energized but you shouldn't do it if you're not.

[erobillard]

So far I've been bringing the parts that were serviced over time back to their original specs, with the assumption the values were whatever the tech had on-hand that were "close enough."

Now I see that the 4x 100k plate resistors on the 6CG7's were quite intentionally replaced with 4x 10k's, highlighted in this partial schematic:


You can see them in this photo, the tan bodies stick out:


I've only found one relevant thread (here) where it was suggested that the plate resistors on a 1484 should be better than 1/2 watt to survive plate voltage jumps over 300V. Any thoughts on why the values would be decreased with the same V rating?

Last caps arrived yesterday, though work and life takes over for the next few days and not sure when I'll be able to finish this off. Also launching an electronics meetup in Toronto starting in October (in case anyone reading is local).

[g1]

I would think the person that put those 10K's where 100K's should be made an error with the colour codes.