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Post by leonski on Aug 5, 2021 1:22:15 GMT -5
In both the VTA ST70 and ST120, the weak point is that pesky RECTIFIER tube. Amps both come with a 5AR4 of modern manufacture. NOS can be had, but at a premium and than from places like Tubes-R-Us or whatever.
I was going over this in one of the guitar amp forums, a real neat place for tube information......and found a schematic which ADDS 2 DIODES to the circuits One in The tube only uses 4 of the 8 pin 'octal' socket...with a one-way KEY so tube only goes
in ONE WAY. the 4 wires are 2x for the 'heater'....this is from the heater output windings of the transformer and 2 from the high voltage output which is what gets rectified and 'sent on'....
Problem is this tube can sometimes ARC which can cause other problems and sometime $$$ problems.....Other times it's just a PIA. But to PREVENT? Add a pair of diodes. In this case? 1N4007 which is a 1000 PIV (peak inverse voltage) diode is added to
each output leg. You UNSOLDER the pair of wires on pins 4 and 6 and solder the CATHODE end of the Diode to that pin. That is the end of the diode with thee BAND. Than loop the diode ot an UNUSED pin.....from 4 to 5 and from 6 to 7 and attach the wires removed to those
new pins. DONE....
This is referenced in Half a Dozen posts across as many forums. And even a Vid on YouTube. If I build a 'kit', this will be an origianal feature so I don't go thru ANY of that nasty stuff with arcing rectifier tubes.
DavidR, above, says a 5AR4 MIGHT go 10,000 hours. I'll bet this is for a NOS tube, probably not current production. but the Diode Mod will help get the MOST out of this tube so you probably don't need to go the expense of a NOS rectifier. At Tube Depot?
I see 'em for 20$ to 190$....the last? Mullard NOS stuff with the box written in Hieroglyphics. I even see an AMPEREX Bugle Boy for an astronomical 400$......But If I thought I needed THAT, I'd shoot myself.
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Post by audiobill on Aug 5, 2021 2:45:33 GMT -5
The diode mod is a well known and documented mod for Bob’s amps. See the Dynaco forum for specific details.
Cheers,
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Post by DavidR on Aug 5, 2021 9:28:51 GMT -5
Its called the yellow sheet mod.
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KeithL
Administrator
Posts: 9,961
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Post by KeithL on Aug 5, 2021 10:38:05 GMT -5
I would add one note to folks that have other similar amplifiers....
Solid state diodes have a lower voltage drop than a tube rectifier and of course don't have a filament that requires power or needs to warm up.
Eliminating the significant amount of heater current some rectifiers tubes require may mean that your power transformer runs a bit cooler.
And, because of their lower forward voltage drop, when you replace a rectifier tube with solid state diodes, you will raise the voltage output on the high voltage supply... You will also lower the output impedance of the power supply - which may otherwise improve performance or alter the way the amp sounds.
Specifically the increase in voltage may increase the output power on the amplifier... And it WILL require you to readjust the bias on some tubes...
However, if the high voltage on your amplifier was already near the limits for your output tubes, or of the high voltage filter capacitors... Replacing the rectifier tube with diodes could cause you to exceed their maximum limits... This could cause the output tubes to fail prematurely or could cause the filter caps to fail outright... (When making modifications like this it's never a bad idea to check the high voltage on the terminals of the power supply caps with a meter... CAREFULLY.)
Also note that, with a tube rectifier, which warms up slowly, the rectifier and the output tubes tend to warm up at similar rates. So, since the voltage of the power supply, and the load presented by the output tubes, come up together, they tend to smoothly balance out. But, when you replace the rectifier tube with diodes, the high voltage may "jump up" briefly when you turn the amplifier on... (The high voltage supply comes up immediately, and may rise somewhat abruptly, until the output tubes warm up and load it down again.)
You need to be sure that this spike in voltage doesn't significantly exceed any maximum ratings - or it could cause the amp to fail prematurely.
AS LONG AS THIS IS AN "APPROVED" MODIFICATION FOR YOUR AMPLIFIER THEN THIS SHOULD NOT BE A PROBLEM.
However, you should not make changes like this to other amplifiers that use a tube rectifier unless you know that it's safe, or thoroughly understand how the changes will affect your particular model...
The diode mod is a well known and documented mod for Bob’s amps. See the Dynaco forum for specific details. Cheers,
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Post by audiobill on Aug 5, 2021 10:51:11 GMT -5
Agree, but in this case it is approved by Bob Latino.
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Post by ttocs on Aug 5, 2021 11:00:45 GMT -5
Agree, but in this case it is approved by Bob Latino. And does the BL approval also cover for M-125 amps? Just curious. I use the Weber SS rectifier with the T for slower start, so maybe this mod doesn't apply to me, but curious just the same.
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Post by audiobill on Aug 5, 2021 11:41:15 GMT -5
Hi ttocs, check on the forum.
Bill
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Post by leonski on Aug 5, 2021 15:35:15 GMT -5
I would add one note to folks that have other similar amplifiers....
Solid state diodes have a lower voltage drop than a tube rectifier and of course don't have a filament that requires power or needs to warm up.
Eliminating the significant amount of heater current some rectifiers tubes require may mean that your power transformer runs a bit cooler.
And, because of their lower forward voltage drop, when you replace a rectifier tube with solid state diodes, you will raise the voltage output on the high voltage supply... You will also lower the output impedance of the power supply - which may otherwise improve performance or alter the way the amp sounds.
Specifically the increase in voltage may increase the output power on the amplifier... And it WILL require you to readjust the bias on some tubes...
However, if the high voltage on your amplifier was already near the limits for your output tubes, or of the high voltage filter capacitors... Replacing the rectifier tube with diodes could cause you to exceed their maximum limits... This could cause the output tubes to fail prematurely or could cause the filter caps to fail outright... (When making modifications like this it's never a bad idea to check the high voltage on the terminals of the power supply caps with a meter... CAREFULLY.)
Also note that, with a tube rectifier, which warms up slowly, the rectifier and the output tubes tend to warm up at similar rates. So, since the voltage of the power supply, and the load presented by the output tubes, come up together, they tend to smoothly balance out. But, when you replace the rectifier tube with diodes, the high voltage may "jump up" briefly when you turn the amplifier on... (The high voltage supply comes up immediately, and may rise somewhat abruptly, until the output tubes warm up and load it down again.)
You need to be sure that this spike in voltage doesn't significantly exceed any maximum ratings - or it could cause the amp to fail prematurely.
AS LONG AS THIS IS AN "APPROVED" MODIFICATION FOR YOUR AMPLIFIER THEN THIS SHOULD NOT BE A PROBLEM.
However, you should not make changes like this to other amplifiers that use a tube rectifier unless you know that it's safe, or thoroughly understand how the changes will affect your particular model...
The diode mod is a well known and documented mod for Bob’s amps. See the Dynaco forum for specific details. Cheers, INSTALLING a Time Delay Relay to give the heaters 15 to 20 seconds to come 'up' before applying high voltage is a GOOD IDEA when using a SS rectifier. Also? Bob L. has writtent that the original amp was calibrated for 110vac LINE voltage. The higher voltage common today....my house is NEVER below 115, unless we are on the verge of a brownout, may effect the high voltage seen by the amp... I'd never heard it called the 'yellow sheet' mod, but it makes good sense. If I build and amp? This will be built in from the start.
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Post by DavidR on Aug 5, 2021 16:13:17 GMT -5
INSTALLING a Time Delay Relay to give the heaters 15 to 20 seconds to come 'up' before applying high voltage is a GOOD IDEA when using a SS rectifier. Also? Bob L. has writtent that the original amp was calibrated for 110vac LINE voltage. The higher voltage common today....my house is NEVER below 115, unless we are on the verge of a brownout, may effect the high voltage seen by the amp... I'd never heard it called the 'yellow sheet' mod, but it makes good sense. If I build and amp? This will be built in from the start. You don't have to use a SS rectifier with the TDR. I use a tube rectifier (Tung Sol) and have had it in place for 3 years with zero issues.
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Post by audiobill on Aug 5, 2021 18:59:31 GMT -5
When he says 110, he means the original dynaco st70 from the ‘60’s, not his current versions, leonski, buy and build with confidence.
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Post by leonski on Aug 5, 2021 19:37:14 GMT -5
You know me well enough now to know I'm on the Very Edge of OCD. It has been a form of fun looking into this. If I like the KLIPSCH speakers, I'll get the ST70 and call it good. Maybe get KT66 tubes? And certainly replace the 12AU7 in the 'center' position with something else. Add the Diodes.....certainly cost <1$ to do.
TDR goes WITH the SS rectifier. I consider it a 'package deal'.....Of Course, you can run the TD relay without the SS rectifier! But the TDR preserves the Easy option...
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Post by garbulky on Aug 5, 2021 21:38:53 GMT -5
Fab tube tech talk guys! Love it!
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Post by leonski on Aug 5, 2021 23:04:23 GMT -5
Lot of OTHER good information about tubes in general....ans epcific types....over on sites catering to GUITAR AMP GUYS.
they argue endlessly about THIS or THAT being better, the sound it makes and so forth. I thought HiFi and HT guys had issues.
They, at least, don't generally arugue about cables.
Look up 'EL34 V 6550' for some entertainment.
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KeithL
Administrator
Posts: 9,961
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Post by KeithL on Aug 6, 2021 9:53:06 GMT -5
Bear in mind that "hi-fi" equipment at least theoretically attempts to deliver clean sound and low distortion... This means that, for example, the driver and output tubes will normally be biased into their linear operating range...
In contrast, with a guitar amp, the amp is supposed to introduce various types and levels of distortion and coloration... You cannot say which one is or is not "good" or "accurate" since the amplifier is part of the instrument - and so is producing rather than reproducing the sound.
Therefore, in many cases, tubes are deliberately operated outside of their most linear operating range. For example, they may deliberately under-bias or overdrive a tube, so that it produces more distortion or a particular type of distortion. And even different tubes that act quite similarly under normal conditions may sound quite different outside of their normal operating range.
And, as we say, while there is only one right, there are an infinite number of ways to be wrong... but, with a guitar amp, all of those are just "right in different ways". (In other words, in a guitar amp, you are quite often deliberately doing things that would be wrong in a high fidelity amplifier.)
Also note that, in many older guitars, the output from the pickups is a very high impedance source... And, often, with a guitar, you may then be connected to the amplifier through significantly long lengths of shielded cable, which may have quite a bit of capacitance... And, in that case, it's quite possible that different cables WILL have a significant impact on what that guitar sounds like.
Lot of OTHER good information about tubes in general....ans epcific types....over on sites catering to GUITAR AMP GUYS. they argue endlessly about THIS or THAT being better, the sound it makes and so forth. I thought HiFi and HT guys had issues. They, at least, don't generally arugue about cables. Look up 'EL34 V 6550' for some entertainment.
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Post by leonski on Aug 7, 2021 1:22:02 GMT -5
While what Keith says is overall true, you should go read what the guitar amp guys have to say about several of the various tubes which are in Common with the Latino amps.
Among them are EL34, KT66, KT88, 6550, maybe KT120 This is for the ST70 and ST120, mainly.
You'll find some patterns of agreement. This regardlessof INTENT of the application. And it gets confusing, in the sense that on only a few point is there substantial agreement.
I found it interesting but NOT the end game comparison when I finally realized they were talkng stuff like Marshall amps or old Fender or even Peevy.....
The essential difference in goal...Reproduction VS Production lend differenes to what is desired.
I'd be curious to compare a decent guitar amp to the ST series amps as a SCHEMATIC.
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Post by DavidR on Nov 4, 2021 14:47:48 GMT -5
Does anyone have experience with the new Psvane 12AU7 tubes? Wondering if they are worth the price and how they might compare to RCA Cleartops or some of the NOS 12AU7 variants. click to enlarge
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Post by pedrocols on Nov 4, 2021 19:14:21 GMT -5
I have been running my M-125 amps for nearly 6 years now trouble free. I have always used SS rectifier without TRD and I did the Diode Mod (Yellow Sheet Mod)
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Post by DavidR on Nov 5, 2021 14:44:35 GMT -5
Although not an amp, below are my tube rolling experience and thoughts thus far with various 6SN7 tubes in my Schiit Freya plus. I haven't found a tube that I didn't particularly like; however, some sounded more refined and or "color" the sound better than others... I'll do my best to describe what I heard. Before taking the plunge, I combed through various Freya tube rolling forum posts as I wanted to become more acquainted with the 6SN7 offerings and gain experience from other owners. Most owners indicated that the right pair of tubes have the most impact to the overall "sound" quality; therefore, quad sets aren't required; matched pairs are recommended though. After reading various posts and messaging a couple of Freya owners, I decided to venture down the rabbit hole... My observations are listed based on what was provided with the Freya and then in order of purchase / delivery. Besides the Tung-Sols and Electro-Harmonix quad matched sets, the rest were matched pairs installed on the right side of the Feya. Tung-Sol 6SN7GTB - Stock tubesDynamic, punchy with less of that "tube" sound, if that makes sense. I've seen / heard Youtubers call the sound signature of the Tung-Sols as the "new" tube sound, not warm; however, certainly not clinical nor cold sounding. They provide just enough "color", yet are different when comparing the sound to older tubes. They provide a wide soundstage, yet it is not as deep as some of the other tubes I have on hand. All in all a great sounding tube, nothing particularly "magical", just a nice sounding tube that is readily available at a decent price point. Electro-Harmonix 6SN7 - Matched Quad Set - Tube Depot - $23.95 / eachI enjoyed their 6922 in my Schiit Vali headphone amp, so these were the 1st tubes that I purchased for the Freya. They are extremely similar to the Tung-Sols; however, have less width to the sounds stage as they sounded ever so slightly "boxed in" in comparison. Don't get me wrong, the soundstage didn't suck; the presentation from the Tung-Sols was simply wider. If I didn't have the Tung-Sols to compare them to, these would be perfect as far as an inexpensive set of readily available tubes. GE 6SN7GTA with copper grid posts, "D" getter; 1950s - Tube Depot - $159 / matched pairSlightly less bass than the Tung-Sols; however, they are much warmer, so I'm starting to hear the "color" that I prefer. Call it distortion if preferred, but I'm starting to hear the "magic" of tubes and those sonic qualities weren't present with the Tung-Sols. The GEs have the same dynamics, impact, and wide soundstage of the Tung-Sols, so I decided to keep them on the left side, while testing the other tubes I have. No particular sound quality reason for that decision, I felt since they were also old / "NOS" tubes that they should pair better with the rest of the tubes I had purchased. RAYTHEON 6SN7GTB short bottle with side "D" getter and black plates - eBay - $38.93 / matched pairAfter letting them cook for a few days, I finally heard the "magic" I was looking for. They have a wider, deeper and taller soundstage. Smooth vocals, fantastic guitar... simply a better sounding tube. There is more presence to the music... just all around better. I regards to soundstage depth, it sounded as though aspects of the soundscape came from behind the speakers.... as in, a few feet behind the speakers... the speakers simply disappeared... they added more dimension to the sound than what I previously heard with the other tubes. With the other tubes the speakers also disappeared, but they weren't as dimensional nor deep as these. RCA 6SN7GTB with bottom getter and black plates; one dated May 1961, the other Nov 1963 - eBay - $47.93 / matched pairNice sounding tubes, but a step down from the Raytheon; these are more like the GEs, but not as refined. They still put a smile to my face, but the Raytheon's are superior. I'll need to do a longer A/B test, but after an quick comparison, I prefer the GEs over the RCAs as the GEs had more detail and presence to the musical presentation. CBS 6SN7GTB with 2-Hole black plates and bottom "D" getter; 1956 Week 22 - eBay - $59.33 / matched pairInitially reminded me of the Raytheon's, but after a quick A/B, they sounded more like the GEs; however, with deeper bass and although less than the Raytheon's, they have more dimension to the soundscape than the GEs presented. These are much warmer than the Tung-Sols and Electro-Harmonix, but not as "magical" as the Raytheon's. I prefer these over the GEs. I'll probably keep these on the left side once I finish listening to all the tubes I have; possibly as a round 2 type test. I have these on hand, just haven't gotten to them yet. Psvane UK-6SN7 factory matched - Grant Fidelity - $119.95 / factory matched pair Raytheon 6SN7GTB medium bottle with black plates, halo getter - eBay - $57.53 / matched pair Sylvania 6SN7GTA with 3 hole black plates; 1954 Week 13 - eBay - $89.93 / matched pair RCA 6C8G with 6SL7 adapter and cable - eBay - $92.50 matched pair Raytheon / Baldwin 6SN7GTB - eBay - $62 matched pair After listening to 6 different tubes, new and "NOS", below is my order of preference; however, as older tubes become more expensive and more difficult to find, I have two options for relatively inexpensive, great sounding and readily available tubes in the Tung-Sols and Electro-Harmonix. RAYTHEON 6SN7GTB short bottle CBS 6SN7GTB GE 6SN7GTA RCA 6SN7GTB Tung-Sols Electro-Harmonix Currently cooking the Sylvania 6SN7GTA with 3 hole black plates; 1954 Week 13. For more info on my room and the decision to head back to 2 channel stereo please visit here: emotivalounge.proboards.com/thread/57382/heading-back-2-channel@luisv, looking for your thoughts on these 6SN7 tubes. Have you had a chance to review them all? Is the Raytheon 6SN7GTB short bottle still your fav?
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Post by pedrocols on Nov 11, 2021 9:15:24 GMT -5
Does anyone have experience with the new Psvane 12AU7 tubes? Wondering if they are worth the price and how they might compare to RCA Cleartops or some of the NOS 12AU7 variants. My favorite 12au7 is the Baldwin. I have tried GE, RCA and Raytheon.
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Post by DavidR on Nov 11, 2021 9:41:07 GMT -5
My favorite 12au7 is the Baldwin. I have tried GE, RCA and Raytheon. I have a pair of these but haven't tried them yet. They are actually RCA Cleartops.
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