Discussion in 'Tube Audio' started by njcanuck, Jan 28, 2017.
Well damn. What do you know?!? The question becomes, what causes that resistor to go?
My suspicion is heat but whether from tubes or current, I don't know? If it goes again, I may replace it with a higher wattage.
Keep us posted! I may just add a couple 1 watters to my next Mouser order.
Does that one have a lot of voltage across it? Possible its beyond what the ratings of that part are meant to deal with.
Based on the schematic voltages given by Dynaco, there is 208 vdc across R5 (the plate resistor associated with the AF Amplifier stage). Current draw through the resistor is just over .5 mA, which equates to a dissipation level of just 0.144 watt. These conditions are hardly stressful for a component rated for .5 watt dissipation -- it is operating at less than 30% of its rating, which is quite conservative.
Moving to a 1 watt resistor might in fact solve the problem. But I suggest that more likely, the resolve that moving to a 1 watt device really brings to the table, is one of moving to a different series of component, rather than moving to a higher rated component (even though that is occurring as well). If Trav and NJ are both using the same component, and both having the same problem with that component, and the component is operating so conservatively for its ratings under the conditions indicated, then its quite appropriate at this point to question the component itself at least as much if not more than what might be damaging the component in the amplifier. Dale is a good brand with an historically good reputation to date. But with all the outsourcing going on today, there may just be quality issues they need to address as well. Bottom line, there is little that the amplifier can do to "damage" this resistor.
I was thinking maybe a voltage rating limitation causing it to break down internally, not so much a dissipation thing.
An excellent thought Gadget, but honestly, even the seemingly micro size half watt resistors of today typically carry a 300 volt rating (for the voltage across them), with the more classically sized units carrying at least a 350 volt rating or higher. Therefore, I cannot imagine that any of this component's specifications are being challenged, causing it to fail. I would again suggest that it is a manufacturing concern with the part in question.
I agree with Dave on the quality of the part being the issue as you can look at today's TV's as a really good example! Today's vehicles!
I don't know what resistor Trav is using but I doubt that it is the same. I'm thinking that in my case the resistor may have been noisy from installation but I had not run the amp long enough to "cook" things in. The resistor I used is a Vishay-Dale metal film 300K. I measure 226V (dropped to 221V over 2hrs) across it at line voltage 124VAC. It is rated for 350V / 1/2W. Temp rating is -65C to +175C. My temp probe shows it operating at 56C, 10 degrees higher than the left channel but the right location is inboard vs outboard on the left. I guess heat shouldn't be a problem!
So far it's holding up. Time will tell ....
Just finished reading this thread, it was very interesting with lots of good to great info! I'm currently running an ST-35 Clone using the Triode stainless chassis (approx. ST-70 size), Shannon Parks' circuit boards (DIYTube), a Dynakit power transformer and two original Dynaco Z565 OPT's. My friend Art, who helped me build the amp, made some pcb's for Dave's EFB circuit as I wanted to take advantage of individual biasing. Plus I'd been "gathering" 7189/EL84/6BQ5 tubes for a while as this project was on the back burner for at least a few years now and have a set of E84L Tele's and a set of Tele EL84 round plates besides some nice, strong Amperex sets. I love it and the fact that it didn't have 4 ohm taps didn't bother me at all as I'm currently playing it through a pair of 15 ohm speakers and have another pair I plan to have cabinets built for and I doubt I'll be using any other speakers so it works well for me.
Always good to hear about another ST-35 project. Sounds like you have the best of both worlds - old and new.
In hindsight, I didn't need to worry about 4 ohm taps either. My 4 ohm Altecs sound better on the 8 ohm taps. Go figure . . .
I've had this little amp burning for two days now and it is completely silent - no static at all. So it seems it was just one noisy resistor. The replacement was the same Dale as the original. All is good now.
Hope it lasts for you! Mine seem to go after a few months. I've used a couple different brands of 1/2 W resistors in that spot. Googling issues with the ST-35 and that resistor seems to indicate that we aren't the only ones that have issues there. Odd!
Hmmm . . . . didn't realize you were experiencing months between failures. I'll circle back to this thread in six months with an update.
As promised, I'm posting the six-month update on this little amp.
Good news: The replacement 300K resistor is holding up just fine - the amp is dead silent. I guess the first resistor was a rare bad one?
I also managed to snag a rare pair of CBS 7247s on eBay for a reasonable price. These are the ones that member Sony6060 often says are the best he has ever heard. They tested strong so I pulled my nos Mullards and stuck them in. It took me quite a bit of back and forth substitution to get a handle on the differences. To my old ears, the CBS tubes may have a very slight advantage in the high frequencies while the Mullards had a definite advantage in the bass dept. In the end, I thought the Mullards presented a better overall balance and stayed with them. Both sets sounded really good, however.
The amp has become one of my favorites and sees daily use. It is rock solid. I think anyone who builds one with Dave's excellent boards will be well pleased.
I did a total rebuild with Dave’s boards and recommended parts list as well. Also using it daily and nary an issue. Love it!
@njcanuck This looks like a great project, thanks for posting! Do you think someone could complete the build with basic tools? I have DVM, VTVM, solder station, hand tools, and some "old" knowledge and skill, but I do not have a signal generator or scope.
Yes. Absolutely. All you need to build it.
I'm looking at it closely! I am not sure how to decide about the 4/8 ohm OPT's. All I have now is 8 ohm Klipcsh bookshelf speakers. And if I decide to go with njcanuck's transformers, is there a different product number? I only find one set of transformers at dynakit for the ST-35 and they don't specify
Not sure if I’m interpreting your question correctly, but there are two output connections for speakers on the back: 4 and 8 Ohm. The OPTs typically support both.
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