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Post by jeffinmn on Dec 13, 2018 7:53:23 GMT
Lately I noticed that the compression circuit wasn't very effective in my V200. Some records were quiet and others very loud. Turning the compressor off made the sound get a little softer but it was consistent from record to record. First I swapped the 6SK7 from my spare known good amp with out a change. Then I swapped the 6SL7GT and compression circuit worked great. When I tested the 6SL7 tube in my Hickok tube tester the first section tested great at 1500. Minimum to be good is 1000. The second section was totally dead. No movement on the meter at all. In all my years of working on tube equipment I've never come upon a tube that had one section perfect and the other section 0. The tube is an RCA JAN (joint army/navy) tube and those are usually very sturdy and reliable. I'm wondering if anything in the Seeburg amp could have caused this or it's just bad luck. The amp has been recapped.
The replacement tube is a newer style RCA tube with the new logo. Both sections test at 1800.
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Post by robnyc on Dec 13, 2018 10:01:54 GMT
It is just a fluke. There is nothing in the amp that would result in that happening. The only thing i suggest is reflowing each pin associated with the defective section. It is rare but not unheard-of to have a cold joint in the tube pin. This was one of the arguments in favor of direct connection in 7 & 9 pin as well as the later designs.
Over the years I've had a number of tubes with burned out heaters. It happens in systems where they are used in continuous service. Those are the types of service calls tech's like -easy.
RobNYC
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Post by Ron Rich on Dec 13, 2018 14:21:00 GMT
Hi Jeff, I have also "tested" tubes and found one section that is dead-- never thought of it, but I'll bet Rob is correct--open connection in the pin is most likely here-- Ron Rich
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Post by jeffinmn on Dec 13, 2018 15:15:20 GMT
Do either of you have a preference for metal or glass tubes especially for the 6SK7 tube?
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Post by Ron Rich on Dec 13, 2018 15:24:39 GMT
Hi Jeff,
I ainna seen no "metal" 6SK7, in many a moon ? I think that was spec'ed as a G, or GB, on that amp- ?? Ron Rich
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Post by jeffinmn on Dec 13, 2018 21:30:38 GMT
In my stash of new/old stock tubes, I have more metal 6SK7s than glass. On both of my HFMAI-L6 amps the tube socket is labeled 6SK7/6SK7GT.
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Post by robnyc on Dec 14, 2018 0:31:02 GMT
Jeff, glass or metal -doesn't matter. What matters if no heater-cathode (H-K)leakage. If there is leakage the tube will cause hum when it conducts heavily such as coming out of squelch or during a "loud" record.
Same is true for the 6BJ6 and any time a tube is used as audio shunt.
RobNYC
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Post by jeffinmn on Dec 15, 2018 21:02:10 GMT
 I ordered a couple of New old stock General Electric brand 6SL7GT tubes which arrived today. They are a newer style and test really strong. On my calibrated Hickok 539B tube tester, mutual conductance should be at 1000 to be considered good. A NOS RCA tube that I already had in stock that I replaced that bad tube with tested at 1800 on both sections. The new GE tubes tests at 2800. Likewise the 6SK7 tube I have in the amp should test at 1260 to be "good" but the one I have in the amp tests at 1900. Is it better to use tubes that have higher mutual conductance (above the minimum to be considered "good") and if so, will one notice a performance difference? In the case of the 6SK7 and 6SL7GT, will stronger tubes allow the amp to bring really quiet or low volume records up to a fixed volume so they aren't as quiet? Conversely will stronger tubes be able to control and compress the sound of really loud records compared to tubes just testing as good? |
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Post by Ron Rich on Dec 15, 2018 22:01:28 GMT
Scott,
Tube "brands" are somewhat meaningless. Just about the same as gasoline today. In this case RCA was owned by GE for years-- same plant probably made both brands of tubes. As to if one rated higher, in MC, will preform better then another --who knows--most likely will vary with each tube, and the condition of the surrounding circuitry. Ron Rich
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Post by robnyc on Dec 16, 2018 5:03:27 GMT
Jeff, the design of the Seeburg tube AGC minimizes differences and wear in the tubes. The AGC sample point is after the reactance tube (SK7 or BJ6). The creates a current feedback loop. If the SK7 or BJ6 begins to weaken and not attenuate the added level gets sampled by the SL7 which in-turn will feed more control voltage to the SK7.
The downside of the circuit becomes obvious if the G1 voltage on the reactance tube becomes greater than voltage on the cathode. When that happens the G1 starts to draw grid current and overloads the very high impedance control supply. This leads to the circuity losing control and the level "running away' and it becomes phasey, loud and distorted.
Since I need maximum potential gain control range for commercial applications, I have to carefully trim the input drive so as to never exceed the critical point. I use the last section of "With Or Without You"-U2 since that is on all machines and becomes extremely bassy and loud at the end. measuring the control voltage is tricky since even the 10 meg ohm load of a DVM will load the circuit and reduce the voltage a bit. My old 1988 B&K reacts fast enough (it is not autorange)and I can get a quick hit reading before the voltage drifts down under it's load.
Max voltage for BJ6 is 25VDC and depending on other mods, 45 VDC for SK7. Reading taken at the time constant 1 mfd cap.
RobNYC
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