option-up Posted February 3, 2021 Posted February 3, 2021 (edited) So just before Christmas I took the plunge and dipped my toes into Valve amplification, buying a Cayin MT-45 integrated amplifier from China-Hifi-Audio online. It arrived promptly: beautifully packed, very shiny and very nicely built. I love it. It also sounds very good driving some Monitor Audio Silver 300s I picked up second-hand on the forums here. So now I've been thinking about tube rolling, tackling the preamp valves first since that's supposed to have the most effect on sound quality. Based on the info sheet of the website I bought it from, I thought the preamp valves were a pair of 12AU7s and 12AX7s, but that was the previous model of my amp. In the new, "Plus" model I was sent, Cayin have changed all of these for 4 x 5BK7A valves (they have done these on a few of the "Plus" models in their range). 5BK7As are also 9 pin twin triodes, branded General Electric rather than Cayin, and presumably NOS. Searching online, these valves seem pretty obscure, mostly used in TVs and radios, with nothing in current production. Has anyone heard of these valves? I asked one online valve seller and they described them as oddball and obsolete. Beats me why Cayin decided to use them over more readily available 12A... variants, unless they have a truckload of them they're trying to use up. I feel like my options for tube rolling are a bit limited here. I assume I can't put in the 12AU7/12AX7 tubes used in the previous model because the heater voltage and current is different (lower) for 5BK7As, so I guess my other option is to get the amp modified to use 12AU7/12AX7s instead? In Melbourne, can anyone recommend who to go to for such mods? Any advice here would be appreciated. Edited February 3, 2021 by option-up Fixed typos and grammatical errors.
aussievintage Posted February 3, 2021 Posted February 3, 2021 34 minutes ago, option-up said: I asked one online valve seller and they described them as oddball and obsolete. Beats me why Cayin decided to use them over more readily available 12A... variants, unless they have a truckload of them they're trying to use up. I feel like my options for tube rolling are a bit limited here. I assume I can't put in the 12AU7/12AX7 tubes used in the previous model because the heater voltage and current is different (lower) for 5BK7As, so I guess my other option is to get the amp modified to use 12AU7/12AX7s instead? In Melbourne, can anyone recommend who to go to for such mods? Yes, very strange choice of valves. One problem is the 4.7 volt heater in these. If the heaters are DC, that means the regulator needs to be altered. If they are ac, maybe it's a simple as wiring the 6.3V ac from the other valves back to the preamp valve sockets. Without looking at all the other parameters, don't know if other changes to the circuit will be needed as well. Using these oddball valves smells of attempted lock-in. Or else they bought metric sh*tload of them somewhere for nothing. 2
muon* Posted February 3, 2021 Posted February 3, 2021 (edited) The 5BK7A may sound rather good and why they used them, lots of TV valves are very good when used for audio if done well, from what I gather. Edit: certainly makes for some cheap tube rolling between different NOS brands a breeze. Maybe Kirk is familiar with these @xlr8or Edited February 3, 2021 by muon* 3
aussievintage Posted February 3, 2021 Posted February 3, 2021 (edited) 7 minutes ago, muon* said: The 5BK7A may sound rather good and why they used them, lots of TV valves are very good when used for audio if done well, from what I gather. I know. I have tried quite a variety myself at different times. I don't think there really is justification for using the more obscure heater voltage types though. There is a 6BK7a version that they might have chosen, for example. Edited February 3, 2021 by aussievintage 2
option-up Posted February 3, 2021 Author Posted February 3, 2021 (edited) Thanks for the thoughts so far. Here's the page on the Cayin website about the amp: https://cn.cayin.cn/shop/products_info?itemid=45 If you scroll quite far down you'll see a side by side photo of the "the old" 12A valves on the left and "the new" GE 5BK7A valves in the Plus version on the right. Google Translate says the pre-amp tubes have been upgraded for a more "vivid and vivid sound, ... which is more cohesive and powerful". Separately, I have emailed Cayin and asked for a circuit diagram. I have the EL34 version, by the way. Edited February 3, 2021 by option-up
RCAJack Posted February 3, 2021 Posted February 3, 2021 (edited) I would see if Cayin can send you a copy of the schematic for the Plus model and the previous model. It would help a tech out to see all the differences laid out, as far as giving you a quote. (edit: oops, you have already ) The question is whether the cost of changing the circuit for the sake of tube rolling is really worth it, considering you like the sound as it is, and you've now got cheap, golden era US made tubes to pick from. It's still an odd valve for Cayin to pick and I agree it makes sense that they got them for peanuts and found the performance was way beyond the Chinese valves they were using before... Or is it conceivable they're really a similar Chinese or Russian valve that's been rebranded? It might be a possibility although it doesn't really make financial sense for Cayin to bother IMO. Edited February 3, 2021 by RCAJack phrasing 1
option-up Posted February 3, 2021 Author Posted February 3, 2021 Cayin's service department got back to me pretty quickly. Because I didn't buy it from an authorised seller, they can't offer support. They said circuit diagrams are for "authorized agents for service reference" so I doubt they would send them to a customer anyway. Well, that's not surprising. The "under the hood" photo on Cayin's website (link above) seems to show a lot. In any case, I just bought two matched pairs of NOS CBS brand 5BK7As off eBay. I'll see how they sound when they get here in a couple of weeks. I was planning on investigating options for modifying this amp (upgrading caps etc.) down the track anyway, so I'll wait until then to ask about the pre-amp valves.
muon* Posted February 3, 2021 Posted February 3, 2021 At the low cost of these valve types get some Raytheon and others too! some black plate types ect 1
xlr8or Posted February 3, 2021 Posted February 3, 2021 (edited) What a real bugger of a tube to use in circuit. It's quite limited in tube rolling options as only 1960's USA branded 5BK7, 5BK7A and 5BK7B tubes are available. The heater supply needs to be changed from 5 volts to 6.3 volts to allow the following tubes to be substituted as straight drop-ins. Talk to Dallas from Dallas Clark Audio in having the amp modded with some Tentlabs modules to up the heater voltage. Edited February 3, 2021 by xlr8or 1
xlr8or Posted February 3, 2021 Posted February 3, 2021 (edited) 1 hour ago, option-up said: Cayin's service department got back to me pretty quickly. Because I didn't buy it from an authorised seller, they can't offer support. They said circuit diagrams are for "authorized agents for service reference" so I doubt they would send them to a customer anyway. Well, that's not surprising. The "under the hood" photo on Cayin's website (link above) seems to show a lot. Tell them you want to become an authorised agent to gain access. It's worth a shot as it sometimes works. Alternatively, ask a mate here to do it for you. Edited February 3, 2021 by xlr8or 1
xlr8or Posted February 3, 2021 Posted February 3, 2021 (edited) 2 hours ago, option-up said: In any case, I just bought two matched pairs of NOS CBS brand 5BK7As off eBay. I'll see how they sound when they get here in a couple of weeks. CBS Hytron or Tung-Sol 5BK7's non A and B versions would be the ones I would recommend as they would be the earliest production variants. I also suspect that most will have identical internal structures meaning they were mostly likely manufactured in the same factory but outsourced and relabelled as different brands. Edited February 3, 2021 by xlr8or 2
option-up Posted February 3, 2021 Author Posted February 3, 2021 Thanks for the replies and recommendations, everyone. Searching online, plenty of people have singles, but matched pairs not so much. And they are nearly always A variants. BangyBang Tubes on eBay has a decent selection of pairs, but they're a whole lot more expensive than nearly everyone else (who often list them under $5 USD each). And postage for 2 from them is upwards of $40 AUD. 1
xlr8or Posted February 3, 2021 Posted February 3, 2021 @option-up Please share a couple of close up photos of the power transformer inside the unit showing the secondary voltages available. My hunch is that both 5 volt and 6.3 volt secondaries are available.
aussievintage Posted February 3, 2021 Posted February 3, 2021 17 minutes ago, xlr8or said: @option-up Please share a couple of close up photos of the power transformer inside the unit showing the secondary voltages available. My hunch is that both 5 volt and 6.3 volt secondaries are available. Must have 6.3 available given the power tubes? Question in my mind is whether the 4.7 for the preamp tubes is tapped ac or regulated dc, or even just ac with a resistor. 2
xlr8or Posted February 3, 2021 Posted February 3, 2021 (edited) 2 hours ago, aussievintage said: Must have 6.3 available given the power tubes? Question in my mind is whether the 4.7 for the preamp tubes is tapped ac or regulated dc, or even just ac with a resistor. Yep - exactly my thinking as well. I'm leaning towards AC tapped or AC with the resistor. Edited February 3, 2021 by xlr8or
RCAJack Posted February 3, 2021 Posted February 3, 2021 Or just 6.3v for everything? Those old TV tubes were built to take abuse. 1
RCAJack Posted February 3, 2021 Posted February 3, 2021 50 minutes ago, option-up said: Thanks for the replies and recommendations, everyone. Searching online, plenty of people have singles, but matched pairs not so much. And they are nearly always A variants. BangyBang Tubes on eBay has a decent selection of pairs, but they're a whole lot more expensive than nearly everyone else (who often list them under $5 USD each). And postage for 2 from them is upwards of $40 AUD. I was looking for some other valves a while back and found the BangyBang prices to be pretty OTT. Maybe you could pick up a few extra 5BK7 singles from tube depot (or whomever) and get them tested. Then just cherry pick the best. But it’d pretty nice to have more options if it only takes some minor tweaking of the filament supply.
xlr8or Posted February 3, 2021 Posted February 3, 2021 The 5 volt is probably there for standard rectifier use in other models. In this specific case the 5BK7 looks to be using this tap. 1
option-up Posted February 3, 2021 Author Posted February 3, 2021 Ok, no chance of photos tonight. But I should be able to pull the bottom off it tomorrow night and post some pics. 3
option-up Posted February 4, 2021 Author Posted February 4, 2021 OK, here are some photos of the amplifier's guts. I guess I really didn't know what to look for, but having a guess, the orange/grey twisted wires tapped from the power transformer are for the preamp valves heaters, and the mud brown twisted wires go to the power valve heaters. Is there anyway to tell what the voltages for each of these lines are? 2
xlr8or Posted February 4, 2021 Posted February 4, 2021 (edited) Okay - I've had a quick look and it appears there are 2 x 100 ohm resistors going to ground off the 9-pin mini 5 volt heaters to create an artificial centre tap. What is of concern though is the 220 volt power transformer that has been used. Getting 4.7 volts from the 5 volt heaters doesn't sound right unless you have 220 volt AC mains coming in. I gather you have somewhere near 240 volts coming in or are using a 240 volt to 220 volt step down? Yes - you can measure the heater voltages on the secondaries if you are experienced in knowing how to take these measurements. Otherwise, a big big caution against going anywhere near there as you will likely fry yourself and the amp. A simple fix is to connect the orange/grey twisted wires to the same taps as the mud brown wires to use the 6BQ7 and other equivalents. @aussievintage can advise any further info as I'm practising way outside my own expertise here. Edited February 4, 2021 by xlr8or 2
option-up Posted February 4, 2021 Author Posted February 4, 2021 (edited) Thanks for the advice @xlr8or. I'm not using any step-down transformer. I thought Aussie power was 230V anyway (used to be 240V but not anymore) and so the transformer is operating within its tolerance of +/- 10% (according to the website spec). I'll wait until I talk to someone about upgrades; I'll see what the eBay 5BK7A's I bought sound like first, and no I'm not going to poke around the internals with a multimeter! Edited February 4, 2021 by option-up 1
muon* Posted February 4, 2021 Posted February 4, 2021 (edited) I used to get a steady 250vac at the mains here, and have heard of people getting up to 260vac in some places or a bit higher, the 230vac is only on paper as a stat. Only way to know is measure at the outlet, you can buy plug in meters that will tell you what it is for around 25 bucks. https://www.bunnings.com.au/arlec-energy-cost-electrical-meter_p4410270 Edited February 4, 2021 by muon* 2
option-up Posted February 4, 2021 Author Posted February 4, 2021 Good idea @muon*. Thanks. I'll check on the weekend. 1
RCAJack Posted February 4, 2021 Posted February 4, 2021 2 hours ago, xlr8or said: A simple fix is to connect the orange/grey twisted wires to the same taps as the mud brown wires to use the 6BQ7 and other equivalents. That may be risky territory for 6.3V secondary. One 6BQ7 will need around .4A, so the four of them will be adding ~1.6A of load to the 6A needed by the EL34s. I'd want concrete data on the transformer rating before going there. Sure is a neatly constructed amp, may I say. 3
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